I guess I am overly careful, but I won't eat or feed a rabbit with a known health issue that could be pathogenic in nature and that would include abscesses with an unknown cause. Although people usually get them from something like Staph, rabbits can get them from all sorts of different oxygen-needing bacteria including Pasteurella, Strep, Pseudomonas, Staph + a whole host of bacteria that don't need oxygen. I am taking microbiology right now for nursing school and all I can say is there are a lot of bugs in the world that can make us sick if they get past our external defenses that may normally not bother us at all. It might just look like an abscess to the naked eye, but it is possible the blood, filtering organs (liver/spleen in particular), or the other tissues like muscle have been infected with whatever caused the problem. The bun could have been on the way to septic before you butchered, etc.- there really is no way to know, but I would rather be safe than sorry.
Best of luck!
Lauren