Like the guy above me said, I'm out of luck with Google Voice. I was thinking of something else though.
I know there are a few applications that backup text messages to an email address. Would creating a separate email address (so my phone wouldn't get the notifications) and automatically backing up incoming messages to the new email address be a ghetto workaround? You could add the 2nd email address to the tablet and turn off the forwarding on the phone at any given time.
The reason for the ambiguous "is this a question or an explanation" tone here is that my Droid X apparently has bugs with every application that I wanted to try this out with. I'm going to check out the Droid X forums, but I thought I would bring it up here.