Michi,
Do you hold that there is a halakhic prohibition here?
If so, what is it?
What is its source?
What is its force?

Why not? This is old material, and you can find it with a simple web search.

Doesn’t the prohibition derive only from Kabbalistic sources?
As the Hatam Sofer said: “Anyone who mixes Kabbalah into Jewish law is liable for sowing mixed kinds…”
In short, maybe this is Kitniyot 2? (something unfounded)

It isn’t only Kabbalistic. See Niddah 13b. I referred you to Google it.

Since the prohibition of wasting seed is that one spills it not in the place intended for it, meaning the womb, and since homosexuals are exempt from the commandment of procreation (that’s what I’ve read in a few places, and if I’m not mistaken that is also your view), it occurred to me to ask this question..

Where did you get the idea that homosexuals are exempt from this commandment? Is there some exposition to that effect that I somehow never knew about?

From the verses in Genesis it seems that the whole problem of wasting seed is this: because he did not want to raise up offspring for his brother. But for an unmarried man, who in any case is not raising up offspring, is there a problem?
And if not, then aside from the time of ovulation it should always be forbidden. And what would you say about a woman who uses a cloth, which is permitted?

There is room to interpret it that way, but that is not the view of the halakhic decisors. A woman who uses a cloth is no different from intercourse on days without ovulation. When there is intercourse, they permit intercourse even when it does not lead to procreation.