Note also that PanMESS claims to support the CPC464plus and CPC6128plus.  Those machines will run /most/ CPC464 software fine, as I understand it, plus they'll run GX4000 carts.  FWIW, I can't get any CPCplus discs or carts running in Caprice32.
I say PanMESS claims this since I can't get it to work yet.  I've given it the bios, but I can't figure out the directory name for Amstrad CPCplus images.  Also, the documentation link is broken.