Ooh, Ooh! I've got one. From an interview with ASM2 producers Avi Arad and Matt Tolmach on the Playlist. Arad was asked about Wolverine and Beast making an appearance in a Spider-Man film:
" Now, for Sony and Fox to make this deal and for Disney to endorse it... We'll probably have peace in the Middle East before that."
Now, why the heck would Disney have to endorse a private business arrangement between Sony and FOX, unless crossovers were not written into any of these contracts?
Let's summarize!
1) The Marvel licensing agreements with FOX and Sony are all separate contracts.
2) It is highly unlikely that crossover language is contained in these agreements signed back in the 1990s.
3) If crossover language is not included in the contracts, neither party has any idea about payment terms, character reversion, etc. as a result of a crossover.
4) If FOX or Sony attempted a crossover with the licensor's permission, Marvel could sue to have FOX or Sony found in violation of the agreements and ask the courts to tear up the contracts.
5) Avi Arad, Zak Penn and Tom Breevort, insiders all, have given their opinion that crossovers require Mama Marvel's permission.