if it's simply a matter of holding the throttle open for long periods of time you might investigate something like this for motorcycles -
http://www.lockitt.com/AccessoriesOther.htm
throttle rocker is clever way to hold throttle open with basically your hand. could easily form something with mild steel/alloy to fit your eBike grip to test the principle.
coming from a motorcycle background I always assumed I would prefer twist throttles on my LEV's. turns out I was wrong - if there's a choice I'd go for thumb throttle any day. one of those weird things that you never know until you try...