That was a good idea to try wiggling the bolts in the cap holes but I think I would probably do your other tip: take a rubber mallet or wood and gently tap the caps where the bolts attached. Tap one side, then the other, working back and forth until it breaks loose. Using a rubber mallet should be gentle on the aluminum. Still, I would not bash on it too hard. Just use some gentle but forceful tapping back and forth, front side and back side. If you cannot get the mallet head in there, you might use a wooden dowel pin to reach the cap. If it still won't come out then back up and revisit what might be holding it.
BTW, was it leaking oil back there?