Below are the pictures that somehow document how I cleaned the board in question. I added some explanations directly to the attachments.
I, by the way, only did one cleaning session, as the PCB was really clean after the first run, which I could only really see after the PCB was completely dry, which didn't take very long, as the whole process did not involve a lot of water.
The steam cleaner I used looked very similar to the one you can see in this picture
, although mine (or better: my parents') is red. The vapour stream the steam cleaner produced really was rather dry, because the steam was so hot and was released under a rather high pressure, I think. Nonetheless, I could regulate the steam pretty well, simply by varying the distance from the nozzle (I didn't use any kind of attached brush, only the steam) to the parts I wanted to clean. Keeping a high distance, I could easily blow off the remaining dust (yes, the steam was dry and fast enough to blow off the dust and did not moisten the dust in a way that it stuck to the PCB), while at a low distance, the steam cleaner almost behaved as some kind of high-pressure water blaster, although it only released a bit of liquid water when I didn't pay enough attention (after a couple of seconds of not using the steam cleaner, some water gathered at/in the tip of the nozzle due to steam cooling down).
After the first run of steam cleaning and after letting the whole thing dry for about 3 days, I detected some dirt that didn't come off. The dirt stuck to the switches, but only in a very loose way, so it could be brushed off by a paintbrush. That there still was some dirt left was (I'm almost certain of that) the result of the cleaning method itself. As I could only blow the dirt around on the PCB, some dirt naturally managed to hide in some gaps, but since this dirt didn't have this cement-like consistence any more, that was not really a problem. I could have probably got rid of it with the steam cleaner, as well, but I simply oversaw some dirt due to poor lighting conditions and the dirt transforming into some kind of dark jelly when it was unloosened by the steam.
So all in all, I think this method of keyboard cleaning has great potential, although I cannot really say a lot about the effects the steam cleaning had on the switches, as some of the switches were sticky before. Apart from that, I think the switches in question were stickier before the cleaning and I could open some switches after the cleaning that I didn't manage to open before, so maybe there is even a positive effect on sticky switches.
Oh, and last but not least, if there are any questions, feel free to ask.
Keyboard science, bitches!