Ahah, you're on to something there, if only it wasn't so late in the afternoon I could think more clearly 
We actually do use work libraries anyway at the moment, copying everything over the top on the original library before checking-in. The cross-testing (first stage testing) is done using the work libraries and the PB IDE. As you say we could also use them for the build for QA, only actually checking in for the production build.
However I suspect QA would not be too happy about that, as they hadn't tested the checked-in code, and I can understand that concern - after all we often have to merge code changes on check-in and mistakes have ocurred in the past.
I shall do some more thinking when my brain is working better
Any other ideas appreciated.