This looks like it might work if you can find it in stock somewhere:
http://www.cdw.com/shop/products/default.aspx?EDC=513590
Then there is this:
http://www.pixela.co.jp/en/products/tv_capture/pix_uvcd_u1w/spec.html
And this:
http://cgi.ebay.co.uk/Firewire-IEEE-1394-6-Pin-F-to-USB-M-Adaptor-Convertor_W0QQitemZ360205865090QQcmdZViewItemQQptZUK_Computing_CablesConnectors_RL?hash=item53ddf15082
And this:
http://cgi.ebay.co.uk/Firewire-IEEE-1394-6-Pin-F-to-USB-M-Adaptor-Convertor_W0QQitemZ270459078925QQcmdZViewItemQQptZUK_Computing_CablesConnectors_RL?hash=item3ef89e090d
And so on:
http://cgi.ebay.co.uk/Firewire-IEEE-1394-6-Pin-F-to-USB-F-Adaptor-Convertor_W0QQitemZ270424332782QQcmdZViewItemQQptZUK_Computing_CablesConnectors_RL?hash=item3ef68bd9ee
http://cgi.ebay.co.uk/Firewire-IEEE-1394-6Pin-M-to-USB-Male-Adaptor-Convertor_W0QQitemZ270424110158QQcmdZViewItemQQptZUK_Computing_CablesConnectors_RL?hash=item3ef688744e
http://cgi.ebay.co.uk/USB-A-Male-M-to-Firewire-IEEE-4Pin-Convertor-Adapter-UK_W0QQitemZ250535495682QQcmdZViewItemQQptZUK_Computing_USB_Cables?hash=item3a55144802
http://cgi.ebay.co.uk/Firewire-IEEE-1394-6-Pin-F-to-USB-F-Adaptor-Convertor_W0QQitemZ270416070517QQcmdZViewItemQQptZUK_Computing_CablesConnectors_RL?hash=item3ef60dc775
Whether it will 'work' is unknown - you might be on your own for driver and hacking - but if you're trying to build your own custom penkit for hacking Windows machines over Firewire then such issues as driver support should be moot. You'd probably be competent enough to hack up your own driver and i/o support for such a case, no?