Now I'm confused. I thought when you mentioned HFST as a re-implementation of the Xerox lexc/xfst, that you were referring to Mans Hulden's foma implementation (http://dingo.sbs.arizona.edu/~mhulden/hulden_foma_2009.pdf, http://wiki.apertium.org/wiki/Foma). Yes, I know, "foma" doesn't look anything like "HFST"...that's what I get for not checking my references first!
My current confusion is whether HFST supports the xfst notion of "replace rules," i.e. more or less normal phonological rules that get composed with an FST (usually on the surface side) and modify the network, e.g.
b --> p / __ # At a quick glance, I don't see those in the pages at the above link to the HFST documentation. Does HFST implement them? They're pretty central to the way most people use the Xerox xfst.