I do something very similar.yeah, i do use a bit of 12 string effect on my piezo acoustic patch. i use two pitch blocks in parallel. the first is a detune (one voice only) with the low cut at about 400hz and the second is a fixed pitch at +12 (one voice only) with the high cut at 2000hz. the idea of using the cuts is to try and make the shifts audible on the lower strings only and the detune on the higher strings only. kinda works ok if it's not mixed in too loud. remember to run parallel blocks at 100% mix.
For the detune I put an envelope modifier on the pitch and vary from 0-9 cents detune based on picking strength.
The idea being that the harder you dig in on a real 12 string the more out of tune the string pairs sound.