Is it possible to run Recovery Manager Forest Edition on a server that is member of a different Active Directory forest?
Yes, it is possible to run Recovery Manager Forest Edition on machine from different forest or even on machine from workgroup. No trust relationships required because you have an opportunity to specify account/password.
Why it is possible?
1. Recovery Manager for Active Directory Computer Collection contains options for specifying account and password for accessing domain.
2. Recovery Manager Forest Edition has ability to specify account/password for access backup and Domain Controllers.