Actually, larger mirrors are possible for LMPs according to this article:
http://auto-racing.speedtv.com/artic...owed-for-2012/
LMP1 diesels will have a theoretical 6-7% power reduction, 5 liters less fuel, and all LMP1, -2, and -C cars will have the dorsal fin next year (as mentioned).
And all LMP(?) and GT cars will have larger rearview mirrors and all GT cars must have a rearview monitor, which is optional for closed LMPs.
It seems that this article only covers general terms, though, and there's probably more specific info elsewhere, but those seem to be the highlights.