
The Harmonium can be played with almost no wind reservoir (usually drawing the E (expression) stop. This enables the player to control the tone (and volume) as they play by the way they press the feeder (pump) pedals. There are a few players in the UK (Jonathan Scott and Anne Page come to mind) who use this technique in some of their concerts and recordings. The sound of the Harmonium can be much gentler than the usual reed organ sound!
I think that none of the Hauptwerk "Harmonium" sample sets have this functionality. I think they are for the most part American Organs (and generally suck air in through the reeds to make the sound rather than blowing the air through the reeds).
Pipeloops has a "Positiv Organ from 1740" sample set, which allows the pipes to be blown by using the expression pedal(s) as a foot operated feeder (http://www.pipeloops.com//product_info.php?cPath=23&products_id=52 click Features). Interesting to try...
I wonder if any of the sample set producers could provide a really Harmonium? Have a look at http://www.jonathanscott.co.uk/mustel-harmonium.htm to see a lovely instrument.
Iain