One option for a guitar player is to use an octave foot pedal (e.g., Roland PK-5) to specify a bass note or part of a chord.

One attractive combination is a Karma with the MOSS option installed driven by a Yamaha WX-5 wind controller and a Roland PK-5 Octave foot pedal. The melody and a single bass note would be sufficient to define a useable autoaccompaniment. However, this method would not work with all arrangers; for example, it would not work with the PSR-8000, since it requires three notes to define a chord. The Karma can be set up to respond to a single note or any combination of notes as a chord. Thus, it provides a chord progression in response to a MIDI melody. If the bass note is added, it defines the root of the chord. I think you can see that if you can accompany a Wind Controller melody, you can also handle a guitar lead through chord changes.