Thanks Doc!
Unfortunately the gameport to usb thing didn't work, XP just didn't recognize the pedals on the USB port.
Doesn't matter anymore though, I tried something different. I hardwired the pedals into the throttle controller in place of the rocker switch. Works like a charm, the X45 doesn't even know anything has changed.
And since pedals and X45 are one unit, I can even use both in older games that don't support multiple controller IDs.
Of course, doing this voided my warranty, but that's a lot better than spending another 150 euros on USB pedals or flying with a rocker switch...
Thanks for your replies guys.