Hi Andrew
To make sure, I have just used a fully functional Realband .seq file (alll MIDI tracks) and added a VSTi which I know works (Kitcore drum) - I use the "Add VSTi plugin" option. Kitcore works in the GM "playback" mode.
If I try to load Amlitube Fender or NI Guitar Rig 3, I get the error message popup "VST plugin was not loaded, either due to canceling or loading a NON VSTI -DLL" (I'm not shouting just copying the error message exactly!).

I hope this helps - other programs like Jamstix will load and the GM "playback" mode will be functional but not the "brain" (where it should do it's own thing at the tempo of the song).
Thank you
Roy