could I mount a toilet on it for a shower toilet combination?
No. You cannot put anything on top of the wet room tray because it's sloped on the entire surface. This is a tray, that must be tiled on and should serve only as shower area.
If you wanted to mount toilet or some furniture you could go for smaller wet room tray and use tile backer board on the remaining floor area. It would be even and could be used for putting down other bathroom fixtures.
I am happy to assist if you have any further questions.
Adam 020 7193 0953