Bowline Alternative
 

To save space, I tie an alternate to the bowline. I believe I learned of it from Brion Toss, but I cannot find it in the 'Loop' section of the knot book. Anyway, to tie the knot, I take a turn around the standing part and then a turn inside the loop thus formed. Drawn up, it resembles two half hitches that got confused so one turn goes the other way. At any rate, once drawn up, it does not slip and it is easy to untie. For my life, though, I cannot remember the name of this wonderful knot, and I want to recommend it by email to a friend using an ATN Top Climber where his bowline prevents him getting as high up the mast as he needs to go.
What is it called?

Buntline?
 

I think maybe you are referring to a buntline hitch? It's a good hitch but not a fixed loop knot like a bowline. Maybe try the "Locked" or "Yosemite Turn" Bowline. I've never seen one of these shake out. I see now you are talking about life support and reserve any recommendation. I, personally, use the scaffold hitch or the buntline hitch, both with stoppers in the tags.