Beetles
(Struggling mightily to NOT make some kind of pun about Brit songsters from the '60's)
Dermestid beetles is what you want. Pick every scrap of soft tissue off the bone. Neat, tidy, quiet, and they don't bite people. Keep 'em covered; the adults, like most beetles, can fly. We used them in the biology lab in which I was an assistant 100 yrs. ago in college. Available through most live-specimen mail-order houses.
No idea re fire ants except that I would NOT like to take a skull into the house after they'd worked on it--how could you ever make sure you'd gotten every one of them off of/out of every little recess in the ears, sinuses, etc??? Fire ants DO bite people.
Something big like a boar's head will take any colony of insects close to forever (how fast could you eat through a railroad car full of steak, say) and the odor can get pretty fierce. If you have a sheltered outbuilding in which to house your beetle colony, that may not pose a significant problem. You can speed up the process by skinning the head and boiling off as much of the soft tissue as possible before giving the head to the beetles.
My wife would call this a criminal waste. She'd saw the head in half, remove the eyes, boil the meat, brains, soft tissue off the bones, and use the boiled stuff to make head cheese. She's a farm girl, Norweigian/German, and that was what they always did at butchering time. The head cheese was a family favorite.