Quote - You misunderstand. In the script, I evaluated the planar projection and then wrote explicit UV's that implement that projection into the OBJ file. These concepts of UV projection mode and so on are only meaningful while I'm in the scripting environment, dealing with the geometry in the object-oriented system. When I call "WriteOBJ" on the object, it becomes a totally standard OBJ file with no magic at all.

Well that I understand perfectly.
And you really don't need to do the dumbed down version for my benefit. ;-)

Quote -
Many of your responses and issues about compatibility with different apps seem to be all driven from this single mistaken premise - that the high-level features, like Level of Detail, like UV projection modes, etc. are lost if you target this app or that app. No they're not. Once I create the loadable prop, all those options are gone. I can create a new prop that expresses variations on those options, but Poser or Maya or whatever will never see those except as explicit frozen OBJ files with explicit topology, explicit vertex positions, and explicit UV values.

No I acknowledged several pages ago that "baked down" versions of what you're doign would be fine, and even accurately described your process before you did to this extent. In the post where I talked about how other programs deal with paramteric objects and history stacks.
My mistaken premises may have  been mistaken premises, but only because you've been real spotty with information as to how it all works until now. Hence I've asked questions to try to nail it down, not just because I was assuming it was one thing when it was something else.
What you've described above is exactly how some app like Maya would deal with it when exporting a static mesh as OBJ: all history/ability to adjust parameters gone and all projections baked into UVs. And your predefined UVs aren't much different than predefined UVs in mesh or NURBS objects.

So when are we gonna get to see this thing? I'm anxious to open it up in a modeling program and have a look.