I just made an awesome discovery. Too cheap to pony up the big bucks for an FM1909 bulb (and other accessories required with purchase), I did some experimenting with some long life 6V bi-pin bulbs and three AW IMR 26500 in a Mag 2D with Kiu socket. I'm pleased to announce that my gut was right. The 2000hr Hikari 6V 20W G4 bulb, JC5607, like it's big brothers, can handle some serious overdriving.
On three 26500s measuring ~12.4V open pack after testing, this bulb was pulling ~5A with no and no complicated setup, just direct drive.
I don't know what kind of bulb life to expect pushing it so close to the envelope, but at just over a dollar a bulb, I don't care. At 60W of super white light, this setup is a Philips 5761 (and Mag85) killer at least for output.
(On a similar note, the 100 hour Philips 20W 6V G4, PH7388, unlike the 5761, can handle two IMR batteries at full open pack 8.4V and output appears slightly brighter then the 5761 driven at 7.9V without all the hassle of drivers or thermistors or battery conditioning.)