Thanks for the response, but I'm embarrassed :-(
Of course, as soon as I had posted this, I found an answer. I actually
have two MIDI devices available, Microsoft GS Wavetable SW Synth which
works for all other apps, and Microsoft MIDI Mapper which seems too
be the only one that works for Impro-visor. I had never used the MIDI
Mapper before, and never thought to look beyond the SW Synth!!