Is this a workable model? Is it ok to use unity express modules for users at a remote office, and have the, still be part of callmanager?
Yes, CUE can be integrated with CM4.1. See this configuration technote:
CallManager for Cisco Unity Express configuration example.
Note that if CUE is already functional e.g. part of a CUCME solution you will need to re-image CUE and re-apply the license. So all voicemails etc are lost.