I can't speak categorically for all types of accompanying instruments for all types of songs, but there are three people who come to mind immediately from the last two FSGW Getaways:

David E. Scheim, who can back up just about any song - beautifully - on harp

and

Leadfingers, who can back up most anything on whistle, guitar or who knows what?

and

Ron Davies - especially on viola (not to mention his amazing whistling)