the FTB launcher actually have the ability to modify mod-packs for local use. So you already have the option to run custom mod packs through the launcher.
using the "Edit Modpack" button you can disable, enable and even add mods to an existing mod pack.
Sure, but let me tell about my personal server. I have a youtube channel that I record on our own server, and I've just updated from our custom 1.4.5 modpack to a 1.4.6 mindcrack one. Now, we want to use several extra mods. Some small ones, like campfires or the elegant punt (great sailboat mod, btw), other not so small like buildcraft advanced power systems, cogs of the machine of minechem.
Now, my only option with the "edit modpack" is to tell them to have two different download folders, one for standar mindcrack servers, other for our customized mindcrack server, and tell them to remember to switch to the customized one when they want to play in our server, which is less than optimal.
A great way to fix this, and a good feature, would be a "create customized version" button, to clone an official modpack with a different name. Then I could customize this modpack and send the newly renamed and modified clone to my players with our dropbox folder system, or something like that. What do you think?