Doubt you can use preset to save subscene or do it without taking screenshot (with current set of triggers available).
That preset save trigger is for subscene atom (stored inside "..\Custom\Atom\SubScene" not "..\Custom\SubScene").
There is no preset tab on subscene atom, so you can use this to fill in subscene info:
creatorname, sig, savename and few minor things...sadly nothing control related (same way person app.preset doesn't save parented nodes).