It's possible with an external MMU.
They exist(ed) for 80286 systems. PC-MOS/386 supported at least one model.
I think it's also possible to use memory of an UMB card in the A segment, to simulate the MCGA video memory.
Well, at least the 64KB of the page frame.
VGA uses bank-switched memory in higher resolutions (256KB, 4 planes of 64KB each).
320x200 pels in 256c (mode 13h) can fit into 64KB, I suppose.
Leaves the simulation of the CRTC registers and VGA BIOS. A physical device with some logic circuitry may help hogging the VGA/MCGA ports the user software writes to,
then making them readable on another place by the CPU again.
Then, a VGA simulator does apply reformatting/dithering and uses the Hercules as an output
On a 386 or higher, some sort of softmpu could be installed in memory which would simulate VGA/MCGA logic in software, then use Hercules as an output device.
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