Lion Replica of Mountain Lion Server possible?

You could always bind (instead of making it a replica) your Lion server to the OD on the Montain Lion server and run services like file sharing, mail, web, etc.

Sure, a server doesn't have to be an OD replica to utilize OD accounts. You can bind a server to OD just like you bind a client. In my organization, I've done this for years. I have an OD master and a replica for redundancy in case the master machine were to die. Those two servers don't do much other than run DNS and OD. I have a third server that I use to provide services to clients. It is bound to OD and I run Calendar, Contacts, File Sharing, Mail, Messages, Time Machine, and Web on it. All my network user accounts work just fine. If I remember correctly when you bind your server all the services are kerberized to work with the OD you are joining.

Also, I run FileMaker server on that third machine, and due to compatiblity issues with Mountain Lion, I had to leave that install at Lion for a while waiting for an update from FileMaker. I didn't have any troubles with having a Lion server bound to an OD provided by Mountain Lion.