Finally, I modified the recalboxrgbdual.ko driver by forcing 50 Hz. Since I have a Pi2Scart which, unlike the RGB Dual, doesn’t have a 50Hz/60Hz switch, I altered the C source code so that it always runs at 50 Hz, as if I had an RGB Dual set to 50 Hz, then I recompiled it for the version I'm using (Recalbox 9.2.2-pulsar).
It works! Of course, I have to use only PAL ROMs; otherwise, Recalbox will (rightly) switch to 60 Hz mode.
