I am not sure about doubleclick (it will be difficult to make it so that it does not affect the current states layout), but Alt key seems to be free for such purposes.
So in theory it could be possible to make slot objects selection with Alt key, and add/remove slots objects to/from selection with Alt+Shift or something like that, because those keys are free.
Considering selections, I would like to propose same actions for collections as well, when hitting Alt+LMB on Select Active collection icon will select objects in it, and hitting Alt+Shift will add collection’s objects to selection.
This can make easier for example, to untangle filtered collections, in case if filtering revealed some problems with scene consistency.
For example, if filtering selected BMW car parts reveals that some parts are in GRASS collection as well, we will be able to select them directly from CM:
So we can probably use Alt based keys to make possible QCD and CM selections the same.
I think it deserves at least to be mentioned and maybe placed in research list)