Originally Posted by
MrInitialMan
The reason I was thinking derailleurs instead of IGHs is I was worried an IGH wouldn't stand up to the weight.
They may not. Most IGH manufacturers publish a minimum recommended chainring/cog ratio to keep the developed torque within the hub's tolerance. The weight of your pedalcar may indeed exceed that limit.
One possibility is the NuVinci continuous variable transmission:
NuVinciŽ Technology | Fallbrook Technologies Inc.
They are heavy and expensive but they make models rugged enough for your use.