This is a tough one. We've gone through an 8' post, working from both ends.

If I recall correctly (it's been a number of years ago), we leveled the post lengthwise and clamped a leveled sighting board extending out past the end of the post. Had a guy running the drill and aligning the bit side to side by sighting down a chalkline along the posts center and a guy sighting the bit's alignment with the sighting board and saying "up a bit, down, down, good" and so forth.

It worked but and was low tech, but if I had a lot of them to do, I'd be looking to make it a one man job.