We did it! Blackfathom Deep Dish is officially done with "vanilla" World of WarCraft raid content. We'll be pushing to 70 on our next raid night. For anyone looking to do this in the future...
Molten Core was awesome! With about 10 people and heirlooms, you can clear the whole thing. Garr was the only boss that gave us trouble. We wound up just bursting him down and letting the other elementals effectively one-shot people at random.
Blackwing Lair was difficult. We needed a carry for Vaelastrasz and Nefarian because, with only 10 people, we just simply didn't have the DPS to get the job done. I really think this can be done with 15 or so people in heirlooms, though.
Ahn'Qiraj 10 was cake. We basically just walked through it. Ahn'Qiraj 40, on the other hand, was impossible with just 10 people. We had to be carried through basically the entire instance, which was really sad. I think we would have been fine with 20-25 people, but it's definitely impossible with just 10.
So far, the guild has been great (even with our occasional failures). Would definitely recommend others try this!