The original note, as written at the time
In the walls of the bathroom, place winding pipes filled with a fluid that can be heated. The heat from the pipes heats the wall surfaces and the surfaces dry quickly, ensuring a sanitary and comfortable bathroom at all times.
Ongoing development
Winding pipes filled with a heatable fluid, cast into the walls of a bathroom, so that the wall surfaces warm and dry quickly — keeping the room sanitary and comfortable at all times.
This is a small system with an outsized effect, and the reasoning is hygienic rather than luxurious.
Mould in a bathroom grows where water sits, and water sits on cold surfaces. Warm the surface above the dew point and the condensation never forms, so the ecology that needs damp never establishes. You are not killing mould; you are removing its habitat.
The pipes are the cheapest possible version of that. Underfloor heating is standard; wall heating is the same technology turned ninety degrees, and a bathroom is the one room where the walls get wet.
The energy question is the honest one. Heating a wall continuously is wasteful, and in a hot climate it is absurd except where it is not — a bathroom is used in short bursts, and pre-heating a wall for ten minutes before a shower is a very small load compared with heating the water.
The better version runs the pipe off the same circuit as the hot water, so the wall is warm when the shower has been used and cold when it has not. Waste heat, routed into the surface that needs drying.
And the wall becomes a radiator, which means the room needs no other one. That is the economy that makes it worth designing.
Dry the surface, not the air.