Did you fry the tube or balast, what went bad?
After seeing your post I remember I saw some 12v battery operated blacklights like those at Wal-mart, they might have had normal white ones too, though I don't remember seeing any. I was looking for strange stuff for a computer mod then though.
I don't know the brand. Anyway if you fried the tube maybe you can get a new one there cheap.
Would it be possible to reduce the voltage to the lights by the 1.8V? Maybe use a potimeter to adjust it like a dimmer switch. I would think being made for AA batteries they should work at a slightly lower V but then again you would not think they would fry at 1.8v over either. I wonder if you would loose much light, or if the balast will even light them at about 10.5V. Then if you gain 1.8V your only at 12.3 and surely they will handle that, but no garauntee.
By the way, you ever see them taped to CB antennas on trucks? We used to do that and they light up nicely but not fully when we keyed the mic to talk. I think the tubes should work ok at 10.5 if you try it, but getting the power from the ballast might be the problem. If they light off a CB attena they should light off nearly anything, we were only in the 4 watt power ranges. But we did not have anytype of balasts to contend with either.