Putting them in series could actually help even though it's only a doubling of the overall resistance so these...
http://www.ebay.co.uk/itm/10-Stainless- ... _443wt_962
if using a pair and using the whole 500mm length of each one would take 37 - 40 amps - around 500 watts total. I don't know how much current normal heated jets use but that looks practical.
Probably better to have the wire inside the tube so all the heat generated gets into the water but it would need some kind of high temp insulator wound around the wire in a loose spiral to keep it from touching the tube walls. 1 metre of stst wire 1mm in diameter would draw 13 - 15 amps - close to 200 watts. Other resistance wires are available but care would have to be taken to chose one that wouldn't corrode - at least stst is straightforward in that respect.
Yes in principle, and Raspberry Pi rather than Arduino IMHO except I just wouldn't take that approach. I'd use separate electronics for every function rather than bundling it all into a processing block - then if / when something goes wrong it's isolated to one function so determining the problem and fixing the fault would be easier.
The only things I've thought of adding to the BX (apart from adjustable wiper delay which has been solved more than adequately by Mat) would be an audible and visual extra warning linked to the lamps of death - a buzzer that has to be reset manually by pressing a button. The buzzer could also be linked to a circuit that sounds it if an indicator is left on for more than a predetermined amount of time and manually reset if the driver actually intends to leave the indicator running for that long.