Actually you very easily could. All this has a pack dev do is run their eventual changes past Reika so that the person who made the mod and has spent a rather absurd amount of time and effort making something different and balanced in unintuitive ways can tell you if you're going to break his mod horribly, and make note of what they changed in the handbook. That's it. Something that will take you almost no time, and something else you should be doing anyway simply because you modified a massive and fairly complex mod.
The notification in the handbook is so people don't go WHOA! THIS MOD IS AWESOME CUZ YOU CAN MAKE THIS FOREST-CHEWING AXE OUT OF WOOD AND SPITBALLS! and then become extremely disappointed when they try it in another pack and vocalize this displeasure most obnoxiously all over the place. Seriously. Go read all the updates in Reika's thread for a week. Come back here and tell me that doesn't happen. Then imagine that about 50 times worse. Welcome to the Monster Era, and that was with an almost entirely stock RotaryCraft!
The pack they are likely to play is NOT going to be another carefully sculpted single-play-through experience pack ala AgSkies, Regrowth, Material Energy, or Crash Landing, odds are VERY VERY good that it is going to be some variant on a "kitchen sink"; please don't kid yourself, the themed packs aren't what people play long-term, or on most servers. Yes they get a kajillionty downloads, yes they're fun, but they don't provide a good platform for endless play in a diverse community. There's a reason DW20's pack is usually the top downloaded FTB pack for its MC version, and Resonant Rise is absolutely massive, the yogs play on a pack like that too, so did the mindcrack crew, and most streamers and youtubers I see play on packs that most people would class as "kitchen sinks". It might even be added on top of a more popular pack. Many of the people I see in communities who mention playing Reika's stuff have slapped them on top of DW20, RR3, or Infinity. They also were most likely introduced to the mods via Monster.
Even completely unmodified, this is the kind of mod that spawns tutorials, example builds, and tips/tricks videos. Which can actively be harmful in even a single major version later. For instance there was a change to the RF to Shaft conversion engine that made it require active cooling at the mid tiers and higher. If you don't carefully read the handbook and make a build involving these from before that change? Bad things can happen. Failing to treat many of Reika's powerful toys with respect can do things ranging from killing you humorously to making Chernobyl out of your base and the neighboring chunks in a double-digit radius. All of which will inevitably be piled right at Reika's feet. We've seen it happen before, the community is even more hostile towards Reika now so there is no reason not to assume it will not happen again.
Seriously it is both not NEARLY as onerous as most people are making it out to be (I actually have a suggestion for that, by the way), and still protects Reika from a level of vitriol and hate from the community he does NOT deserve, that has been documented, can be verified by dozens of third parties, and can easily be proved.
That idea? Give a list of the things which shouldn't be modified for progression breaking reasons. That list is actually pretty small. The bedrock breaker, all the native engines, the fermenter, the blast furnace, the grinder, the extractor, the furnace heater, tungsten, bedrock dust, and jet fuel. That's pretty much the extent of "don't touch this without really good reasons and being very careful or you're GOING to break the mod", right? Basically everything else could be disabled in most packs without breaking All The Things. I wouldn't suggest disabling the Dew Point Aggregator if you're planning on ReactorCraft either
But just give a list of the things which you shouldn't touch without REALLY careful reasoning, and say both that you're available for discussion if you really want to try modifiying those things on the list and that if it's not on the list, you can probably modify it, but run it past you anyway so you can provide some advice in case of unforseen issues. Then it's not this huge beast where touching something will Call Down The Wrath Of Reika Upon Your Heathen Head; it's just a list of things you should probably avoid messing with, and a little bit of extra stuff to tack ontop of the permissions request... oh and some bookkeeping that basically every user of ANY pack would appreciate if they ever try and play another pack with some of those mods, even one they assemble themselves. I can't tell you the number of times I tried something in Agrarian Skies, EVEN HAVING PLAYED MAGIC FARM!!!! That made me scream at my computer screen "Why is this not working?!", go search on Google if the problem wasn't a crafting recipe or disabled item, and find out "Because Jadedcat changed it". Being introduced to Blood Magic by Ag Skies, I tried to make mob spawn eggs in my next world and couldn't figure out why it wasn't working until I googled it and saw it was an agrarian skies only config. The bee multiplication thing? Ag Skies only. The recipe for Pneumaticraft seeds that was dye+seed? I've seen soo many people try and do that with stock Pneumaticraft after being intro'd to it by Crash Landing.
These carefully crafted experiences are making undocumented changes that are headaches for users all over the place. They SHOULD have these changes documented somewhere, and this is the place so many authors skimp on. It shouldn't just be a line in the quest book, it should be in a place the user cna go and find all these changes in a nice ordered list. Yes it's work, try adding sensible Thaumcraft 4 aspects to every item in a 250-mod pack and documenting it because most of those items have no aspects and I don't want my players getting used to using TiCo Heart Canisters from their mob farms as a source of Victus only to have that trip them up in every other pack ever because they are aspectless. Or have them find a guide that suggests diamonds as an EMC source when I add a mod which gives a plant-based method of diamond creation that gives diamonds an EMC of 4096. Or any number of other headaches that could be avoided if I actually spent the time to let my players know that "hey, this awesome thing I worked on is different from stock. Enjoy it being so awesome, but don't expect it in someone else's pack".