Personally, I wouldn't want to hang one on an RV-12 if I were building. By the time you're finished I'm not sure how much money you'll actually save, but I'm pretty sure you'll significantly extend the build time -- and it will (if history is any indication) be difficult to sell when you need to sell it. It certainly would be "experimental", though.
That said, I do like what I have seen of the Aeromomentum engines and PSRU. I talked to the guy for a while at Airventure in '17. I have another project under way that I may hang one on, if I live long enough to actually get to the point of needing an engine.
RV-12 # 222 N980KM "Screamin' Canary" (bought flying)
Fisher Celebrity (under construction)
Previous RV-7 project (sold)