AES key length greater than 256 bits - is it dangerous to do an implementation outside of the standard?
There is no obvious extension of AES for larger key sizes. At best, AS3Crypto may be hashing the key to 256-bits for you, but the risk that AS3Crypto is doing something horribly insecure in that case is astronomically larger than any benefit that you could gain by having a larger key size.

$2^{256}$ is so huge that the only way that it could ever be brute-forced is by finding a significant AES weakness that would likely make all AES key sizes vulnerable in any case.

Even though you currently don't have any need to use anything other than AS3Crypto, that might change and it would be nice not to have a non-standard cipher in place.

Lastly, if you really do want to exceed AES-256 security (and you are probably fooling yourself, as I think you know), you would be better off XORing with a completely different cipher like Salsa20. Tahoe-LAFS (which is written by people who know what they are doing, crypto-wise) has taken this approach as they are aiming for 100 year security: https://tahoe-lafs.org/trac/tahoe-lafs/wiki/OneHundredYearCryptography.