Finally came to that :

using this abstraction :

But am still interested to know how I could cope with that : just pick up some defined CCs from an unspecified ctlin.

midiselect can select note, cc, pc

don't know if it answers.

I would simply use multiple ctlin objects like [ctlin 16], [ctlin 17] etc.

@Julien : I had a look at midiselect but it doesn't fit what I wanted to do : I wanted to use an unspecified [ctlin] then being able to select CC # at will possibly using this kind of combination :

But after trying different stuff (for example using defined [ctlin] then changing their argument using 'set $1' messages, I came to the conclusion that the most smart way is, like Broc wrote, multiple ctlin objects. Such a shame...