dubdisciple
I have no way of confirming, but I suspect the sonar one may not be transferable. If the one you have is transferable, perhaps transfer that one instead. i can't imagine it would be too hard to tie certain serial numbers to Sonar users.
That's the thing, before you register it it's not a transfer, you just give/sell someone the serial and they register it, at which point it becomes theirs. AD may have some internal note saying it was purchased with Sonar and can't be transferred afterwards (though I seriously doubt it), but they don't actually check for a Sonar registration when you register it.
EDIT: again though, probably not entirely in line with the intent of the deal.