PVA in a bathroom is shite. Any hint of moisture and it'll re-wet making anything else on top, like tiles lift and come off.
How far is the face of the wood from the face of the wall?
Depending on depth, I'd be tempted to tack on some expanded metal mesh to the timber. Treat the wood with a killer/preserver too. Then render level and tank the lot. What's the plan in terms of finish for the walls and floor here?