Unless I am totally overthinking it and missing a simple function... (which I am known to do, all the time). Also, I am assuming this is in ZScript because I don't know any way to access this sort of stuff in Decorate.
You can access information about the current sector of an actor with the CurSector variable, which accesses a sector struct: https://zdoom.org/wiki/Structs:Sector
That page hasn't been updated in a while, but I looked in the full entry from gzdoom.pk3/zscript/mapdata.zs, and in there I can see:
native TextureID GetTexture(int pos);
And from looking in the source for a similar thing earlier, "pos" selects the floor or ceiling - 0 for floor, 1 for ceiling. I don't know exactly how to work with TextureIDs though. Dunno if it can be mapped to a string directly or not.
Also, an important note - something derived from MapSpot probably won't be able to access CurSector as it has the flags NOBLOCKMAP and NOSECTOR
Hope this helps though.