-- it is possible if your boards VIP and PID matches the driver.
if you could provide more details it whould be easier to answer
Is is possible, unless you need ISO transfers. WinUSB handles only bulk and interrupt transfers. But für this the WinUSB is very easy to use, fast and stable. And WinUSB driver is pre-signed by Microsoft, so there is no problem to use the driver with Vista/7 x64 even without going the Winqual way.
WinUSB or any generic driver for that matter should be able to work with FX2LP since it is a generic USB device.
For firmware download you have to A0 vendor request and load the firmware to FX2LP. The format of A0 vendor request to be used is clearly documented in the TRM.