I like the idea.
To get your cargo area lower, you'll probably have to go with a trike, sidecar or trailer design. Sidecar and trailer both have the advantage of being far more stable for loading and unloading. I posted it elsewhere
, but here's a link to Xtracycle
, they have an extension kit system and sidecar that they claim can hold 250 lbs.
Food for thought, if nothing else.