What is the exact reason why you can't have expat (or libxml2 for what matters) installed? Is it a matter of not having root access, or did someone with authority tell you you could not install it?

Not having root access shouldn't stop you: just install it as a regular user, in a place where you have the right, and pass the location when you create the MAkefile (I would guess you would run perl Makefile.PL LDFLAGS=-L/my/path_to_lib INC=-I/my/path_to_header, but I have never tried it). I am sure you can also compile xmlwf (or better, xmllint) statically.