Just a dynamo can run forwards or backwards w/o any problem... but who knows what kinda sneaky stuff they've hidden away in there?
...but - I jus' went and gave my X-FDD a spin ta see, only to learn I've burned out my driving light lol Anyhoo - after replacing that bulb (I've a high beam too) it does indeed work just fine spinning backwards... but I gotta ask, why would you mount it reversed? Dare to be different?
edit: I see you've said "bulbs" which I'll infer means you're running more than just one 6v lamp bulb, which surprisingly the X-FDD doesn't like very much as it doesn't actually put out a lot in the way of amperage. As I'm running a taillight, driving/running light as well as a headlight/high beam I've switched to the LED replacement flashlight bulb Walmart sells, designed to operate at any voltage between 3 & 12v it works well... tho a touch more than I'd like to pay for a flashlight bulb normally lol
At the moment I'm running the LED in just the taillight... but as the driving light jus' quit (cheapo incandescent) I guess I'll be gettin' another heh