Ah, the famous --prefix=/usr! No, of course I hadn't applied that.
> Rebuild with the correct option and reinstall, let us know if that works.
Since I have - duh! - purged the respective kernel since, I cannot do a
rebuild. Sorry, I should have kept it around for testing this out. But
thanks for the hint, it sounds reasonable to me that that's what caused
the problem. I will probably compile my own ALSA driver on another
generic Debian kernel soon and report the results here.