By spray do you mean the "rooster tails" from tires on wet pavement? Possibly flag the track as "wet" but without using wet textures?
I recall years ago when N2k3 modding was allowed, someone was working on a speedboat mod. The "track" was a lake with the course marked by buoys, the water thrown up behind the boat was made by retexturing the dirt/dust effect to water and declaring the track surface as dirt (they also banked the corners so the boats would lean into the turns). So the boats ran on a dirt track with banked turns that looked like a flat lake.
Maybe you'll have to similarly fool GTR2.