Exchange server as Open Directory domain member?

Crazy dream or…?
What do you think, would it be possible to set up an Exchange Server to be an Open Directory domain member and get it’s account information from a Tiger server acting as an Open Directory master? I am in a situation where Macs are generally much preferred (We are an Apple Center), but to make certain things work, Exchange server is needed. Open Xchange is not an option, because stable, mature and well supported system is needed, and the workgroup features must fully support MS Entourage with the soon to be released Service pack that is supposed to bring 100% Exchange server support. BTW, we are licenced to use all Microsoft products free, being a big reseller, so Exchange server costs just the hardware… So even if there is an alternative to the Exchange server, it would have to be free.