Hey sleepyrando, since you aspire to ride the PBP one day, have you contacted your local rando/audax club and asked around? If I were you, I would start with contacting the local rando club and asking as many people as possible (better if they have an online forum for the club) to find out what bike shop they use to build their bikes. I would then go to that shop and describe the predicament. If the shop is pretty good, they would have an adjustable fit bike that is used to determine your fit - frame size, reach etc. Once they know your measurements, they should be able to find a frame, perhaps with 650b wheels, that would fit you, mixte or otherwise.
When I straddle my rando bike, I barely clear the top tube when standing in my shoes. Unless you are a fairly new cyclist, you shouldn't be too concerned about the standover height as long as you can clear it without any pressure on the body while standing in your shoes.