One of the ways that works for me is to use two hands and split the keyboard, transpose the lower section to the same pitch as the upper and pick the guitar sound of choice. Then whang away with your left hand rapidly appoggiaturing (that's rapidly arpeggio-ing in one direction) upwards (for the pick's down stroke) and the right hand playing the same voicing appoggiaturing downwards for the up stroke. You can't easily do full 6 string voicing this way (too wide a chord for one hand!), but the rhythmic energy across 4 or 3 voice chords goes a long way towards giving you the energy that a rhytm guitar has. Perhaps try making the sound of the left hand split a little thicker than the right to emulate the weight that the downstroke usually has.

Have fun, and always practise to a drum beat - there are enough bad rhythm guitarists out there already!!

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There are 10 kinds of people
that understand binary - those
who do and those who don't.