Ok, feedback on Feliway:
We have had the diffusor in a central place of the flat now for a week - we placed it into a socket in the hallway - and there hasn't been an incidence of her peeing! We sprayed some of the pheromones onto her favourite place on the couch too, as the information on the package indicates, and it is working wonders for her: She's so relaxed, purrs so much more, often rubs her head on the fabric of the sofa and keeps it there while looking around a bit upside down, thus really opening up, displaying her throat and chin to be stroked. She's like a baby, calm and revelling in her own beingness - really feeling comfortable.
So, Anna, great suggestion, thanks! I definitely recommend giving this a try when the behaviour becomes conspicuous as in her case.