(i) It doesn't say there must be a figure eight knot any other type of stopper knot, just a knot, so a bowline fit's that category.ii. The tail of the mainsheet may also be knotted
Seems clear cut to me. You must have a stopper knot and may also tie off the tail if you wish.
Reading the rule again, I have an issue with the other end.
"...and be attached to the becket of the aft boom block."
I've never tied my mainsheet to this block, instead I've always run the sheet back along the boom, through the block, forward through the upper traveler block, then back up and forward again through the becket and stopped with a figure eight, not attached to the becket. Hard to go b-to-block (two-blocked) on the sheet if you attach it. It would seem this is not legal.
Thoughts?