There is no way to get terminal services to run correctly on an SBS box. See this SBS blog posting:
The best way I've seen of getting this done is to use SBS premium or SBS standard with a 2008 Standard license. You would install 2008 Standard on the bare metal of your server and virtualize both SBS and Server 2008 Standard. You're allowed in the license of server standard to have 1 install on the bare metal that is only doing the Hyper V role with an additional virtualized install running any roles you'd like. Then enable Terminal Services on the virtual instance of Server 2008 Standard.
This is a fully supported configuration and will do everything you want it to do. The tough part would be P2Ving the current SBS box. This is not impossible, or hard really, just nerve wracking if you're trying to P2V to the same physical box.
As far as any other alternatives, I'm not really sure they'd work. Citrix, for instance, requires Terminal Services before it will function.