The trike guys here on the list can feel free to correct me if I'm wrong, but I think most trikes can be steered one-handed because the two hand grips are tied together one way or another and balance obviously isn't an issue. So the big problem becomes shifting and braking, both of which can be put on a single side. I think the SRAM dual-drive hub puts shifting for both the hub and derailleur on the right grip by default, and brakes can be tied together with a coupler. I recommend going to bentrideronline and checking out their buyer's guide
here. You might get some other ideas.