Hate to be a pain, but, the reason the SegSurrey never went into production is that it violates FDA rules regarding wheelchairs. A wheelchair needs to have more than two wheels. A wheelchair is for a person restricted to a sitting position. Since the SegSaddle can be used standing or sitting it is NOT a wheelchair. However, the SegSurrey, a SegSaddle with a platform instead of a bike seat, can't be used standing up, is therefore a wheelchair, and therefore violates FDA rules. Obviously I didn't know all this when we first designed it, because we designed it to be switchable from one mode to the other.
Now, there is a Swedish solution that is wheel chair height, and does exactly what you want, and since it's made in Sweden it doesn't violate US rules. You just have to import it on your own.
It's called the AddSeat.
http://addmovement.se/en/theaddseat/