Yea I think using 12V would be the better way to go and we have the 31mm festoons on hand in cool white. The advantage of using LEDs is that the power consumption is much lower so you will have very little losses in any power supply.
Regarding the BA9S bulbs, it is important to note the direction the bulb will be facing. We have two types, one with a single 10mm LED where all the light comes out the end, and a 5x LED one with 4 of the LEDs facing out the sides so the light goes in all directions. I am looking at sourcing a 9 LED version of the BA9S which will be brighter, but this will be a while away.
Cheers,
Carl