Very specific Feat ability; DC boost to cantrips

Working off of the Wandslinger Fighting Style feat in Quickstone; Frontiers of Eberron, I wanted to add this to the homebrewed version here on DBB:

"Mystic Force. When you cast a cantrip that requires the target to make a saving throw, increase the DC of the saving throw by 1."

Any idea how I'd go about that?

Hmm...3 days and no insights. 

Damn.

Is this simply too specific to be coded into the DDB Homebrew system?

It's possible to increase spell save DC in general in homebrew, but there's no way to limit that effect to only certain spells (e.g. just cantrips).