I have made a test wad for this feature, I don't know why, but polyobjects that are double sided just make the engine freeze when loading the map, also, I don't know exactly, but polyobjects don't seem to handle sector heights for the start spots neither, so it makes implementation difficult for this, I'm posting my example here Crimson Wizard, you can check it out and try to implement the 3d poly or discuss further your ideas, maybe we can together try to find if this works or not, heh!
You know, Firebrand, after a minute of examining your map I came to thought that these two blocks hanging in the air are not actually polyobjects, but just a plain 3dfloors. Polys stay where you placed them - in the dummy sector. And they look like normal solid polys - stretched from floor to ceiling.
Heh! I managed to get a poly that LOOKS like 3d, (but it doesn't act as 3d).
I created a new subsector in the main map sector (where player is), tagged it with tag 6 (so it appear 3d floor), then moved there a start spot for the 1st polyobject.
When I ran map a suprising thing happened. A poly appeared to be "cut" - without upper partition. It looked like a column, that stretch from floor, but didn't reach ceiling. And render normally! It even moved normally. It stayed normally until I walked up the big at the platform and jumped at the new subsector (in which start spot is placed). Then, a lower part of poly render thru floor. Also, you cannot walk above that poly. Its upper partition is just invisible, but still exist.
Ok, I now know how to create a simulation of 3d platform that stands on floor (and which cannot be walked above, a pity...
1) Create a normal poly, just as you always do.
2) create in your map a subsector for that poly.
3) make that subsector 3dfloor, just always. Its lower surface (actually a ceiling) will "cut" the poly's upper partition. You may make it absolutely thin (a 1 pt of height maybe).
4) make the sector with 3dfloor linedef (sector which defines 3floor properties) absolutely invisible - remove all wall textures and floor textures as well (write "-" symbol in all textures).
5) Result: you receive a block, that stands on floor but do not touch ceiling (of height you choose). You can use it as all other polys. The only bad thing is that you cannot jump on it since it has invisible, but solid walls stretching up to ceiling.
NOTE: I tried to make several 3dfloors in a subsector, so it have a number of "levels", and put poly's start spot at some height, thinking that this will make poly be uprised above floor as well. Unfortunately this didn't happened.
PS. By the way, I just realised you can create invisible 3dfloors and this does not cause any rendering problems
Imaging something like invisible bridges!
PPS. Can anybody explain, where I can place my test/demo maps for download? For some reason I cannot edit my website currently, so I cannot publish it there.