There's really not a way we can test that. As you said, it's in an iFrame, so unless we try to also pull down the avatar images on each page load to detect if it's not working, then we won't be able to tell. Then, if we were to detect a delay, we'd have to test periodically to see if they were working again.. which could cause other, similar delays.
Hope that makes sense.. but obviously, this is one of those issues that there's simply nothing we can do about it. Fortunately, in 3 years of FB development, this is the worst we've seen of this issue.. and it was fixed in about 2 hours (a little longer in some areas), but overall, liveable in our opinion.
Hope that explains, but if you have other suggestions, we're always open and listening!
Alex