Matt, I have several that are derived from the Coltrane 'Giant Steps' 3 tonic system (C E Ab) where I add other familiar formulas like tritone subs and different chord qualities to disguise the sequence.

You can do the same with a 4 tonic system based on m3rd key changes.

I've some that are more like polymodal ideas to apply when the chords are static and/or when I need to come up with changes over a pedal point that support chromatic or intervallic lines.

Other methods I use are not really applicable to the automated chord sub function; it's more a case of using constant structures that follow the melody or counter melody so they're going to be different for every tune.

Alan