Cross forest mailbox migration - retain shortcuts? Advice requested

Hi, I have the following scenario:

A customer has EV2007 deployed in the same forest / domain as Exchange 2003 and is successfully archiving mailbox contents and creating shortcuts to archived content.

This customer is migrating to a managed Exchange 2010 solution, deployed in a resource forest with a one way trust from the account forest for authentication.

The ideal is to migrate the mailbox contents, complete with placeholders, to the Exchange 2010 server so that users can continue to access their archived content in the way they are familiar. There will not necessarily be a requirement to continue archiving from Exchange 2010: just access existing content.

It seems the simplest way forward (though it's highly likely I'm wrong) would be to leave the EV server in the accounts forest, upgrading to EV8 then 9 if required, and giving the VSA the required permissions on the Exchange 2010 so that the mailboxes and their shortcuts are accessible. Only I've tried this in a test setup and can't get it to work! When adding the Exchange 2010 server as an archiving target I'm unable to browse for a system mailbox.

Has anyone else worked on a similar problem? What was your ultimate solution?

Thanks!