Protecting beton cellulaire blocks outside

[Ian]

I have a garden wall - usual construction of mud between stone outer skins. Water got in and half has fallen down. I can't salvage enough stones to rebuild it the same way, so I'm building the back half in beton cellulaire blocks, and then facing it with the remaining stones set in lime mortar.

While BC blocks are acceptable outside, I think the back face will need some protection from the weather (front and top are OK - stone and coping tiles). A coat of enduit isn't possible, as access is difficult - it's my neighbour's garden and she might not like the mess I'd make. Would breathable masonry paint be enough? I could do that fairly easily/cleanly.

Thanks