In this case, my usual pre-coffee tiredness solved the problem:
I had planned to mix the espresso grind with my usual grind to minimize the total amount of grounds in the cup. The idea was
1/2 too-fine grounds -> 1/2 the amount of dregs in the coffee.
Being tired, I only remembered this when I had already filled the moka more than half-way with the too-fine grind, so I just topped it up with the other kind and coooked my coffee.
But lo and behold: There were no coarse grounds in my pot - just some super-fine ones that weren't noticeable. I have since repeated the experiment a few times and can confirm that a top layer of "just right" grounds seems to be able to hold back most of the too-fine crumbs, especially those responsible for the "gritty" mouthfeel.
A top layer of 1/4th to 1/3rd of the correct grind proved sufficient as "filter" in my experiments.
Also, there is no excess clogging of the moka, causing the valve to trigger or hot steam/water escaping round the gasket. The pot works just as usual. Of course one may not compact the coffee grounds in the brewing chamber, but that's standard procedure.