There are many attributes you can sycnhronize between AD/Exchange in a separate forest. Do you want to populate just enough attributes to populate the GAL in the separate forest?
This Technet article covers all the Exchange Related attributes and provides a description on how they are used. You can probably find what you need in there.
http://www.microsoft.com/technet/prodtechnol/exchange/guides/E2k3ADPerm/77b56f0d-bea9-495d-91e9-2d91645e3dfa.mspx?mfr=true
The minimum attributes for provisioning mailboxes are mailNickName, mDBUsedefaults, and homeMDB.