Okay, so here's what I've tried this evening;
Dismounted the brake-pads, sprayed them in with brake cleaner (MOTIP), dried them with a clean towel, lightly sanded the pads with sandpaper, sprayed them again, cleaned them again with a towel. The pads sure felt clean afterwards.
Then I cleaned the rotor with another clean towel, with just some fresh brake cleaner on it. The rotor still felt quite smooth (not meant positively) after cleaning it.
How did my rear-brake performed afterwards?
- Not good, if not a bit more worse than it was. Maybe it's the rotor that needs additional cleaning.
I just re-read Eric Hoefers post and now I'm thinking this brake cleaner may be of a comparable type as the liquid degreaser he dissuaded. :madman:
So I'm more and more leaning towards the use of heat, though I found that one of the more radical idea's.

Unfortunately me, or any of my surroundings do not have a blowtorch. But yes, I do have a gas stove.
I tried to understand the posts here, but due to language barrier (I'm Dutch), I was not able to completely understand what was written.
Here's my question: Do you have to put the pads
in the flame of the gas stove? Or do you have to keep it just right next to it, until all has vaporized? Note that my gas stove only produces this aggresive blue flame and not a regular 'candle'-type flame. Could I use the flame of a candle instead? Grumpy's warnings on explosions scare me a bit.