You probably just need to use a G code with all medicare patients - even if 70 % of your patients you do not e-prescribe at the visit.
I see where the built in e prescribing in AC would let you do e prescribing, but places like Medco don't seem to accept them (can't find them as a listed pharmacy).
The only problem with e prescribing is that pharmacies will electronically send and then you have to remember to look there...Once AC integrates it better it will be well worth the work to do.