G'day NSEU,
All the places I have operated to over the last 7+ years have a stated TL and I can't recall what the FMC comes up with in a place like La Paz. I don't think it matters much because I also can't recall if the TL information is used for any sort of alerting function, certainly not on the B767s I fly at the moment though it does say this, "The FMC uses transition level to change displays between FL and feet" in the B747-400 Ops Manual.
Regards,
BH.