You were faster than me.

the mirrored surface does not want to be translucent in any way. It's either there or it's not, I can't get any options in the middle

I ended up with the same. That's why I render the reflected resp. transmitted light separately, and then compose the result.

I have only dealt with transparency. Translucency is a different story because it involves scattering. If only we can master that (I don't know how) with any non-mirroring material, probably compositing also fits this case.