Hello Roelof,
WinpkFilter drivers works on the bottom of the Windows network stack (below TCP/IP), but application layer of Windows XP firewall works on the top of network stack (otherwise it won’t be able to control applications network access). So I don’t think that there is any easy solution to this problem.
However, may be setting up the virtual network interface, disabling Windows firewall for it and bridging it to the real network interface using WinpkFilter can solve the problem. It’s just the first idea, may be some other tricks are also possible…
Hope it helps…