In my renovation I’ve got 4 sections of floor-ceiling vertical pipe boxing to do. One in the kitchen needs to be water-resistant and part-tiled, part-painted, the other three are just painted.

 

I’m thinking about using something like 12.5mm Marmox multiboard as it’s plasterable and tileable directly and much lighter and more manageable than plasterboard. (the extra cost is insignificant in the amounts I’d need)

 

Anyone else done anything like that?

It would be fine, but a little OTT IMHO. Do you mean moisture-resistant not water-resistant? If so, then green PB would more than suffice; which I use in wet room showers with a layer of tanking solution on top of to create water-resistant shower cubicles ( on 1st floors as well as ground floors ) without issue.