My situation:
I like riding fixed as much as possible
I co-own a bike cargo company

My solution:
turns out there's not much of a problem. I use cargo bikes for super heave stuff (over 200 lbs,) I've got a fixie I leave in the shop for medium loads that can go on the rack or in a trailer (usually a blue sky cyclecart, perfect for anything up to 200 or so) geared nice and low and with 32mm tires for increased traction, and for light stuff I have my high-geared fast fixie. You think riding a fixie is good training? Try riding a fixie while towing 150# uphill!