Any question answered...

I have had success with putting the frame on the ground (on some scrap wood), then using a long lever (big torque wrench).

If the frame is on the floor then the crank is also stopped by said floor, then you just need a long enough lever and a hench-person to hold the frame.

This is a bit tricky because the chainring stops the bb from being able to rest on the floor. But the key there is probably the hench-person. Maybe the builders would like to help?