It seems like there is one fragment within zone .wld files that bears the zone's shortname (e.g. HIGHPASS_MP, on the 0x31 (material fragment list) fragment).
It's
possible that the client searches for this fragment by name and refuses to load any geometry if it can't find it (although it would be a bit silly to search for it by name given other available options).
Expanding a string in the .wld file's string block is non-trivial, unfortunately, although it would be easy to make a tool to do it. Changing "HIGHPASS_MP" to "HIGHPASSHOLD_MP" would be worth trying, at least.
Converting the geometry data from the .s3d version of the zone into the .eqg format
is also an option, although it would be confusing having two different "highpasshold.eqg" files around.