OK. It took a while for this to click for me.
What sjf is politely saying is if you put apiVersion="29" or higher in your mod, anyone using the latest stable version, 1.6.4, from the Kronosaur website or (I assume) Steam can't play your mod. The API for 1.6.4 is 28. Anything higher gives a (no-doubt dreaded by now) "[Requires a newer version of Transendence.exe]" message and disables the selection box.
Thanks, sjf, for pointing this out. I'm working through my mods now and will reupload them to be available to as many game versions as possible. I'll include info on what version or above is required to play the mod so people don't have to download it to find out,
Note that some mods use functions or images that aren't included in earlier game versions. eg the Scorpion GodPlayership obviously needs the Scorpion images so can't be used in version 1.6.4 because the Scorpion images didn't exist then.
Nice work, shane.
EDIT: Of course as soon as I work this out George releases 1.7 as the new stable version.

Back to the drawing board!

Stupid code. Do what I want, not what I typed in!