Interesting discussion! Firstly, in a stable myst world is there a way to create a world with the starting layer at around 64? I made a flatland plains one for a quarry, but the ground level is 89! Unsurprisingly I now have enough scrap from all the cobble and dirt to keep my MF running til next Christmas but not a lot of anything else except some coal and apatite :/ . I see many recommend the mushroom biome, is this level 64 by default or is that 89 as well?
Regarding the dense ore world gens, Abdiel's comment above is a good one about the decay eating the ores before the quarry can get to them - something I hadn't considered tbh, but thinking outside of the box, I'm wondering about the BC filler and whether you could set up the frame quick enough for it to clear out a sizeable area before the decay gets to you? Anyone that watches Etho will have seen how fast his harvests the blocks and you could just toss away the ores you don't want on the ground and keep the ones you do. However, one potential issue I can foresee with this idea is the lava which would cause you a lot of problems if you have it running upwards, especially if it hits a big lava lake. Is there a mystcraft world gen that doesn't have any lava??