I'm in this same situation. Have you had any more luck finding out how to show carplay's screen on the rear screens for example? My kids always love seeing the navigation stuff - so having the carplay display cloned (while I'm in Apple Maps) would be fun... Obviously I wish there were a way to stream HD content to the rear VES monitors instead of crappy RCA, but I'll keep using RCA for now if that's all that's available.
Downside is, if I stream my ipad to the pioneer, it doesn't fill the screen either - so it's RCA quality, and only fills 75% of the screen due to overscan I guess. U