Any question answered...

..But as mdcc suggests...for the additional "stuff" that'd be required I may as well go for several gears. I guess I can always just use 5/6 gears from the cassette stack and jib the rest. A plus is simplicity of the wheel build; the Hope hub mentioned is symmetrical (no dishing). It'd be great to be able to use a thumb shifter with some like this:

Does anyone have any idea of the weight differences between the above:

Hope hub, 5 gears, a short cage derailleur and a thumb shifter/grip shifter

Vs

An internal 3 speed hub with coaster brake?

It's theoretically possibly, in the sense that it's perfectly feasible for an RD to work well shifting a two tooth difference somewhere in the middle of its range, but in most cases by the time you've lengthened the limit screws enough to provide stops at both ends, one or other of them will no longer be aligned with the stop surface they are supposed to act on.

It's pretty pointless exercise, once you've gone down the route of adding an RD, shift lever and cable, you might as well get a cassette hub and enjoy a decent range of gears, even if it's only 5 or 6 on a Hope Trials hub.