I've just checked in a firmware upload utility for the at76c503a. It implements the USB Device Firmware Upgrade specification, with some extra proprietary stuff from Atmel. It works with an OHCI USB controller, but refuses to work on my newer laptop with an UHCI USB controller. I'm not sure WHY it fails, since a co-developer has it working on his laptop with an UHCI USB controller. The code is working (it works on the OHCI controller), but something in my UHCI controller or the linux usb driver or both isn't doing what it's supposed to.
The code can be found in cvs (:pserver:firstname.lastname@example.org:/cvsroot, password cvs, atmel-fw repository), and if anyone has got any idea what may be causing this, I'd love to hear someone else's opinion.