Just a thought, but drawing the bird before hanging it may have two disadvantages. It could encourage dehydration of the flesh, if there weren't the soggy entrails, and it could also permit aerobic bacteria and moulds to enter the cavity and truly foul up the flesh quickly. The process of decomposition from within is slow because it takes place in an environment with little oxygen. Maybe the best answer is to try it...
As for heads or tails, my father loved his pheasant very gamey, so would hang by the head until the bird fell with a soggy thud onto the pantry floor. My mother wasn't best pleased, but as he did the plucking and drawing - and the scrubbing of the floor - she'd tolerate it.
Tried that once and only once. The rotten mass of viscera just rotted its way through the belly skin and ended up on floor. The stench was incredible which took weeks to get rid of.
There was nothing edible on the rotten carcass, and the temp was under 3 degrees for the whole hanging period. Necks were okay though and still hanging.
As for decomposition, this takes place from the inside first (if not flyblown) which is why animals bloat with gas and sometimes explode (as someone on this site can testify to )
So why not draw it before hanging and eliminate some of the risks?
You see I don't understand that hanging by the neck undrawn has any advantages over hanging by the neck drawn.
If you're gonna leave the guts in, you may as well hang it from the feet and get some benefit - see what I'm getting at?
I'd have thought that while you could draw before hanging, the chances of then contaminating the meat with enteric bacteria from the bird are far greater.
And, of course, if you're after some of the enzymes from the gut helping to 'mature' the flavour, then you want to leave the guts in.
What I'm visualising going wrong is a bird that's been shot say by a shotgun, with wounds going into or through the breast, allowing material from the entrails to get into the breast and ruin it. The enzyme molecules themselves will diffuse just fine anyway, but if you hang the bird with pressure for bulk fluid to flow into those wounds then the chances of spoilage would seem to be much greater.
You hang phesants so that when it is cooked it is tenderer. Something to do with fibers in the meat opening up so that it can soak up more juices when cooked. Also never leave pheasants lying on the floor ontop of each other. This causes sweating and taints the meat.