Solid state batteries have different weight and construction which makes a retrofit difficult and could impact the vehicle’s handling. Consequently, solving those problems would be expensive for the end consumer and I really don’t see how a retrofit would make any sense.
I say no benefit because idling the gas engine or running it while driving is a distinction without a difference as far as its end result: charging the battery. However enabling such a system to idle without the driver present creates unique challenges (eg. software, exhaust considerations...