vrkelley: I "assume"
any MR-11 bulb would fit into another MR-11 recepticle and it would just be a matter of getting the wattage I wanted and matching the appropriate battery size for the duration I needed??? I doubt I will be pedaling hard down hill when it's dark ... but even coasting I hit 30 so I really wouldn't want 2-3# of battery swinging around! I thought about either adding extra velcro tape, or just putting the whole battery in my Carradice bag; there's plenty of room, though I may have to extend wiring. I am worried about the higher capacity battery fitting what I visually calculate to be about a 6"x3"x1.5" bag. If it doesn't see above about putting the whole thing in my saddle bag.
Ceiliazul: My understanding is that 6 volt connections have to be more heavy duty than 12 volt as they carry higher amp loads. I also think heat would be related to wattage rather than voltage. But since I would be increasing wattage by 300-400% it is a valid concern. Hoping that cooler ambiant temps and airflow would help with this. Plus the 2nd 6 volt system I pictured appears to have the exact same housing as the 12 volt system you got ... just a single instead of a double. Visually the lamp housing itself appears pretty much identical to the 10 watt 6 & 12 volt ones, just grey instead of black. My gut feeling is that all these housings were originally designed for higher power applications and have been "depowered" for portable battery use on bikes.