I can't imagine how I got this far along and missed the obvious.
My migration plan was to get the Z server in place, create all the accounts, set them all to pass-through to the Exchange server's SMTPin, and then, when everything was working properly, peel accounts off to the Z server one user at a time.
There's just one small, eensy problem.
I don't see how, now that I'm at that point, to convince Exchange to send inhouse mail to those users over to Z.
Can I use the Delivery Options->Alternate Recipient to do that?
I guess I'll try it. Keep it tuned right here, folks.