Hi! In answer to your question, Jos.  I use the flat of my foot to press two or even three adjacent keys, either white, white or white black or white, black, white.  I can play 3rds if one key is white and the other black by turning foot sideways. I play with both feet. 
How did you put a minor switch in your system?  I saw an old arranger system on E-bay that had 2 switches on each side of the volume pedal.  Maj-min-aug-dim if I remember correctly.  It was a large system that included keyboard and pedals.  I can't remember the name but it was one of the major early synthesizer companies.  Could have been Arp or Moog.  Noone even bid on it.
Reflecting on the OMB situation, I remembered that I e-mailed the creator with my question.  I forgot what the question was but remember that the OMB could do what I was wanting.  Perhaps a newer version has or will have the features I need.   
Marcus
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Marcus