I know this sounds weird, but what about a Translucent Transparent FOF at opacity #255 sloped at the edge of those slopes? I hope you know what I mean when I say that. Maybe that will be able to slope the side texture, but I'm not sure.
Well I ran a thought experiment (not an actual experiment, but thinking through the process) and there are a couple reasons why this wont work. First, side textures are tiled normally weather the top of a sector is sloped or not (same goes for FOFs). Secondly, tagged sectors drawn exclusively for FOFs are not sloped in essence. So any middle textures take the shape (sloped or not) of the underlining sector. A flat sector tagged for a sloped FOF will draw a flat middle texture. Thirdly (and this is something that needs to be addressed anyway)
Middle textures' Y offset can not be adjusted.
Again, I found a problem in which middle textures are either stuck to the floor or ceiling of the drawn linedef, depending on where their pegged.
In any and all cases,
all efforts to adjust a middle textures Y offset have failed.
EDIT;
I discovered the pattern of the nonsloping middle textures concerning weather or not they hang over the slope or disappear into it.
I believe in thorough bug reports (it helps if y'all know everything I know)
Anyways, I found that it depends on the nonsloped height of the sloped sector. Let me explain.
Lets say you have 3 sectors in a row, the 2 on either side are at different heights and the middle is smoothly sloped between them.
The middle sector is going to have one of its linedefs set 186, or what have you. Lets say, at first it's the line that joins it to the lower sector of the 3. The slope sector's height now has to be set to that of the higher sector. It is this height that determines where the nonsloping middle texture will rest. In this case it will hang over the slope.
If you rearrange the slope to slope from the higher sector and set its height to that of the lower sector, its nonsloping middle texture will disappear into the slope.
I'm sure y'all know a hell of a lot more than me but I hope we can learn from what I've seen:D