Ignore this, I wasn't aware that "solo in-place" does exactly that.
Just posting here, if someone else stumbles across this.
In the API use "2" for soloing a track with all its routing.
Code:
I_SOLO : int * : 0=not soloed, 1=solo, 2=soloed in place. also: 5=solo-safe solo, 6=solo-safe soloed in place