I think iCtrl-R fits “fully functional, battery operated midi controller” description well. You may want to check with the manufacturer though.
Or if you’re determined to stick with Morningstar MC6 then you may try one of those rechargeable battery packs to power the MC6 through its USB power input and see if that’s reliable enough for your application/setup.
Also, if you insist powering it through its MIDI jack then there’s this Phantom Power Insertion Cable
but there might be potential incompatibilities on the H9 end (like, whether H9’s relevant MIDI pins are not connected to anywhere internally etc.), so this also should be checked with the Eventide to be on the safe side : )