Nick,
Glyn Lancaster-Jones at the traditional boatyard, Port Dinorwic is the guy you need to speak to- he makes most of the two drum types in theSBA, and I think he offers it in kit form too.
His www;
www.traditionalboatshop.co.uk.
The three drum is the better performer, thats widely recognised, quicker to steam and more water content, with a better circulation.
However that is NOT to say the two drum is a bad boiler, as you know, it is in itself a very good boiler design, just in model sizes it won't hold much more than about 300ml so would NEED a feed pump as you mention. Have you thought of using commercially available boiler thermistors as sensors- could be easily added to either three or two-drum design and use minimal voltage with dead accurate and reliable temperature controlled electrical output, and they come with standard thread to put into bushes which could be soldered into the end caps no problem.
However, Geralds suggestion is a very good one, with water level above the steam supply to the pump it would be a hydraulic motor and would run really slowly- how reliable could you get? Only problem I can see is that in a model watertube boiler on the pond, any slight wave could create waterhammer in the pipe as it pitches and rolls and bu**er the valvechest up, however,as you are the model piston valve king, that would be the answer to that particular problem...
I think there would be a market for it- especially for enclosed models where fitting hand feed pumps is a right farce and engine driven ones are hidden at the bottom of an unaccessable deck void. A little hint towards something that might be worth developing to use in steam, it may already have i'm not sure myself. The devices I talk of are the 'automatic air relief valves' that domestic boilers use and are screwed into the top of the heat exchangers . They stay open and release the trapped air in the top of the H.Ex., when the air is released and water touches the valve it automatically shuts the outlet- I think it works on differential pressures between the gas and water. I'm not sure if it relies on being under atmospheric pressure to work, or if condensing steam would stop it working, but if you could link the device to the feed pump steam inlet...
Really looking forward to seeing the LIFU hull!!!! Put me down for one- fit it out like 'Churr' one of the prettiest launches on the Thames, I think I sent you a pic.
Greg