I think you may not understand my thoughts on backbuttering...you do it while setting the tile, not putting a layer on and letting it dry first. I've found that it takes a fair amount of pressure to push a layer of thinset into the pores. Just a once over to get a coat on doesn't cut it. But, if you press well, while wet, the only way to get it off is to scrub it off. You can't scrap it off. I'd guess something like Redgard would make a decent bond and if it works, hey why not.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters),
Mapei Training 2014, Laticrete Workshop 2014, and Longtime Forum Participant.