As I suspected, the new Final Cut Pro 7 Alpha Transition effect was implemented in FxScript, which (as the language hasn't changed) means that it also works in earlier versions. Probably back to version 5.1.
This means that if you know your 7 project might need to be finished on a 6.X system, you can export as XML as usual, and use the Alpha Transition by installing it on the older system.
Note that Apple's license agreement may not allow you to do this (even though the transition is implemented using mere 5.62K of FxScript), but if you use 6 and 7 on your computers and 7 was upgraded from your copy of 6, you are probably bending (if not breaking) the rules already.
@alex4d
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Alexandre Gollner,
Editor, Zone 2-North West, London
alex4d on twitter, facebook, .wordpress.com + .com