You can, but why would you?
Particularly the UltraHDMI is already more capable than the OSSC in terms of scaling (framerate conversion, overscan, gamma boost, CRT bloom, scanlines); and, by converting to analogue to route it through the OSSC, it’ll no longer be a digital-to-digital solution.
If you’re just trying to route multiple devices through an mCable, just connect your devices to one or more Vorke HD41 Pro HDMI switchers (known compatible with the OSSC and up to 4K@60Hz) and use the mCable on the output.
I was thinking on this because of the line multiplier factor, to improve jaggy edges… For that too I was thinking on the Mcable as you can see here:
GCHD Mk-II + mCable = Cleeeaannn! ? pic.twitter.com/dkuFSdhZDw
— EON (@EON_gamingHD) February 5, 2019
Also in this topic you can see something related where the Code Veronica screenshots are: