I added sysex output to usb hosting firmware recently (its how I drive the ableton push LCD) ,
I guess I could add it to midi din/usb device.
(adding sysex input would need more work, and poses some questions)
NRPN/RPN are just CC messages so these can already be sent.
yes can easily be used as a sort of midi translator/router... I use it as such for my virus, as well as a sequencer