Can you point to the code in Facter that does this?  I ask, because the default behaviour of wpeinit is to bring up and DHCP on all the interfaces, and we don't really have any mechanism to distinguish which PXE happened over as far as I can tell.

(I assume that WMI is exposing the same data, by the way, either in the common, or the Win32 namespaces, but whatever.  Once I know what I am looking for I can dig it out.)