I was down at Bunnings in Nunawading today and took the chance to grab a look at their bathroom panelling. They had a bunch of lovely white stuff, but it was all semi-gloss and not at all suitable for use as a screen.
So the answer appears to be 'no' as far as I can tell. Right at the moment I'm still looking at 3mm MDF or ply, 2100x1180, painted with Gesso or maybe Ken's magic mix of the low-gain variety.
- David Eddy