After a night thinking of it.... Some thoughts about expression
1 I admit that expression needs some practice. This feature is mostly ignored by hobby harmonium-players, because this is highly responsive and let hear all what you do with your feet.
2 Why two pedals for expression and not one swell-pedal? Because every dynamic is immediately at hand (at feet), all the time, with 2 pedals (press both feet Ped hard and you get immediately an fff just after a ppp). With a swell-pedal you need all the way between ppp and fff. Even if the effect would be limitated (the fff-sound of a french harmonium is far more metallic than the average sound of a free reed; and the volume range in midi has only 127 values) in a first virtual harmonium, this would be a very interesting feature.
3 I do suppose this is quite easy to do with two swell pedals connected to a micro-controller that analyzes the movements of the pedals
This would make this feature independant from the soundfont and also usable for a VPO and even a Harpsichord
for experiments. Unforunately, I have almost no idea of pic-programming....