Arno, I think those devices could be of great help especially to people with many tanks. In my central array of Paro-tanks 24 of them (each 12 liters) are assembled on a small wall, taking not more space than a compact bookshelf. A photo of this has often been printed, in the Finke-Hallmann-book and here in the forum, too. Another row contains 6 of them, a third 3. Altogether I have 33 small 12-liter-Paro-tanks, lot of work if water change is tried to be practised once a week.
But it would be useful to explain your suggestion in more detail, Arno: What are these devices? You tell us they are used for toilets. But often there is a huge swimming device to be found controlling the water level. I do not see such a thing on your photos. How do your much smaller valves work? I think that they are used for filling the new water in. But before you have to get the old water out? Where have I to look or go for them? How expensive are they? And how do you manage the other end of the pipe, that is: the fresh water reserve? You have to prepare the right soft blackwater and store it somewhere. Is this centrally installed or is it mobile? In a fishroom you can do it the first way, in a living room preferably the second.
Your question: How often? will be answered in different a way. The best answer would be: As often as possible. In nature, all licorice gouramies live in slowly floating waters. Once a week a third would be great. But it’s an ideal, for me it’s far from reality. It works for some tanks but not for all. The living food for a single pair or small young is helpful: The deterioration of the tank water happens much slower than with fish fed on artifical food. But sometimes I need four weeks for a change, I must admit. In some cases there are even longer intervals. I do not recommend that, not at all, but it happens if you have many small tanks; and if the tanks are fed properly it will be tolerated by many species. If you feed very consciously this is mostly (it depends a bit on the species; the most distributed like linkei or filamentosus tolerate that) no problem. In a way you could control the readiness for spawning by this: The more you change, the more they spawn. Exactly for that situation: many small tanks, your devices could help substantially.
Therefore I think that your suggestion could prove very heplful to many of us, but more explanation for the laymen please! Go into the details!