ZDoom

Just an idea: maybe use an actor mover with interpolation points?
Also in ZScript you can give an actor any position you want in some sort of time loop with Actor.SetOrigin.

That worked, at least in this simple scenario. I'm not sure how possible it will be to apply it to a more complicated map with lots of oddly shaped open area; I'm still holding out for a 3d floor solution but now I have something to work with.

I am not sure if this can even be accomplished without sector portals. The issue here is, conveyor belts are properties of a sector, and 3D floors are an anomaly (aka hack) of the system.

Please keep in mind, this is an empirical understanding, so I may be wrong about the technicalities of this - please someone correct me if I am: An actor can only occupy one sector at a time, so they will take the conveyor belt properties of that sector, and if they land on a floor in that sector, 3D floor or otherwise, they will take the properties of the host sector they are in, whereas visually everything appears okay simply because the conveyor belt controls texture offsets and those will be properly transferred to the 3D floor.

There's a demo map [url=https://forum.zdoom.org/viewtopic.php?f=39&t=59149]here[/url] which should help you decipher exactly how to create a sector portal.

I have here, in a vastly oversimplified example of my actual level, a conveyor belt which crosses over itself. The texture scrolling isn't a problem but carrying is; I would like for the barrel in this picture (which starts at "A") to eventually be carried over to position "B". It instead stops on the 3d floor (location "C"). Other people seem to think this is doable but none have said how.
[imgur]https://imgur.com/eQkSpuG[/imgur]

I have tried several things but there are a lot of possible combinations. A vavoom style 3d floor with Scroll_Floor is what I am using above; it makes the most sense and is the simplest. I was surprised it didn't work (plus one of the 'How do I' posts had a similar situation and suggested it; the implication was it worked for them). Sector_SetCurrent didn't have the desired effect on the top section, Sector_SetWind didn't affect the barrel. None of the sector or linedef specials seemed to work either. Does anyone have a working example or know what, if any, combination of 3d sector types and specials, will have the desired effect? There's not too much room for fakery since players beings and things will be all over the level, but I could probably forgive the belts pushing flying creatures or not being entirely seamless if that's the limit.

Example map is attached in case anyone wants to experiment.