When you activated the reinstalls, did you use the key or just your LiveID? I've reinstalled the beta a few times and simply entering the LiveID got it activated once again just as in the released version.
So, if you tried to activate by entering the key again, don't. Just use your LiveID that was used to originally activate..
If you received an error about too many activations and used only your LiveID, then you just need to use that copy in unactivated beta mode.
-steve