double vapor barrier
It looks by what you're saying that it would creeate a double vapor barrier (or vapor pocket), however with the materials shown, it may not create "problems" per se.

That said, I don't know why you would do it using those materials. You could easily grout a tile a shower fairly cheaply and get a more rugged finished product. Personally that's what I would do because it is the standard, easy and correct thing for a shower...

Having said that, you could also just use your liquid sealant layer and heavily coat your motor board and get an equally water-proof later.