I was in bike-brainstorming mode a while ago - before I spotted this thread.
How far did you get?
- It seems possible to fit an Arduino Nano board (http://www.arduino.cc/en/Main/ArduinoBoardNano) inside the handlebar tube. A series of rechargeable AAs could also fit in there.
I would have thought that it is too big. How big is the internal diameter of your handlebars? 
The lithium ion battery I am thinking of using definitely won't fit in though...
- I was thinking of taking some old speed shifter levers (I'm converting one of my bikes to single-speed) and trying to convert them to electrical switches - although I haven't thought of the waterproofing question yet
I don't have any space to fit extra levers on the recumbent I am going to be mounting this on (I made the recumbent too so I may re-make a carbon fibre seat with a space for all of this to be connected which will be difficult to get to with the seat on and will be waterproof).
Regards,
Mowcius