Thanks for all the replies.
Research has been conducted, with much footwork. To summarise, breeze blocks are similar to ytong in properties, but the 'grain' of the bricks I've found is huge, massively so. Some blocks were built of blobs like marbles stacked together, so it's basically useless for ant purposes unless covered with grout anyway.
So, based on that I'm probably going to have to default to grout, Polycell as suggested, with some synthetic hyperabsorbant cloth and crushed silica gel between layers; that stuff is legendary for water retention. It soaks up so much that when saturated, it basically becomes solidly stacked water molecules, and gives them up again when the surroundings dry out. I'll report back on the success of the method, but I anticipate perfect results.
I'm sort of stuck with my outworld at the moment though. I have a square aquarium, so have a lot of height to use with hills, valleys, trees, etc., but I'd rather not flood the base with grout to enable that. I'm unsure what to do there, as a loose substrate is undesirable for many reasons... I'm torn between a shallow, compressable foundation that I can later break through to snap and remove a grout skin on top, allowing complex terraforming with the grout layer, or just using fixed substrate-covered acrylic on the base, for a flat outworld. Suggestions are very welcome.