Your BIOS has to support booting from a USB drive. If so, then all you have to do is format it, do a SYS X:, and copy over your mobo update. SYS.COM is only available on Win9x machines, however, so you'll may need temporary access to one.
I wrote a version that works on Win2k/XP but I won't have access to it until next week It's pretty trivial so I'm surprised no one has done it already.