I've got some simmering now. I used some dog chew hoof pieces I had left from making bow tips. I ran them through my belt sander and caught the dust so there is no collagen in it, just keratin. I'm assuming that if primitive people made this stuff there is no need for pressure cooking. I'm supposed to mix an acid with it after it's boiled a while so I'll use vinegar, they would have had that.
The part that makes me doubt this is that there is so little info out there. If you search for hide glue you're set for three days of reading, hoof glue, maybe 10 minutes.