I think there's just some wrong terminology here, maybe a typo, rabbits can't get Endometriosis. I have a book on lab animals somewhere and it said they had to induce it in rabbits, it only occurs naturally in humans and some primates. They consider that unfortunate because rabbits are cheap, primates are not.
(I would think the rectal version is the same, I thought it was a rare extension of the vaginal kind).
From what I've read, vaginal bleeding in rabbits is most often uterine polyps, uterine cancer or the above-mentioned Endometrial Venous Aneurysms (had to look that one up, but its a pretty common cause of sporadic bleeding).
I'm sure there are other things that can cause rectal (and urinary) bleeding, something else to look into.