In general, a resource manager doesn't need to do anything special about Phase 0; this phase will be handled solely by the transaction manager. But, because in the case of promotable transactions using the PSPE mechanism, the resource manager acts as a special "transaction manager", your resource manager needs to be aware of Phase 0. Why? Because in this case, the resource manager "owns" the distributed transaction and System.Transaction will call Commit on the PSPE interface asking the resource manager to call Commit on the distributed transaction. The resource manager needs to be aware that even after it will call Commit on the distributed transaction it owns, there is a possibility that requests to enlist and do work as part of the same transaction are still possible, based on the Phase 0 rules. And these requests are supposed to be accepted. Don't worry, there will be no data corruption. MSDTC guarantees that Phase 0 happens before Phase 1 starts delivering its "prepare" messages.
What exactly you need to do to ensure Phase 0 support? Let's create the list:
Sounds like a lot of steps, but in reality, this can be said in a short summary: "when you receive IPromotableSinglePhaseNotification::SinglePhaseCommit, pass along the Commit to the distributed transaction you own and behave like it didn't happen :), i.e. accept new connection requests in the same transaction until MSDTC delivers you the "prepare" message.
Of course, if Promote was never called, then there isn't any distributed transaction involved and thus you can proceed with commiting the work when IPromotableSinglePhaseNotification::SinglePhaseCommit is called.
Thanks for supporting promotable transactions and Phase 0.