I used to use a CGA emulator (don't remember which one it was), that could show some gfx even in EGA/VGA-only games (when I selected EGA mode). I remember my frustration seeing some garbled graphics and wanting to fix the emulator, but I had no source code then. Obviously some level of emulation is/was possible.
There can not be an EGA emulator for many reasons.
Emulating CGA works, because in graphics mode, Hercules has a 64k frame buffer at address B0000h - BFFFFh, and for a single page, only 32k from address B0000h is displayed. The CGA has a 16k frame buffer at B8000h - BBFFFh, which fits inside the frame buffer provided by the Hercules card.
Therefore what the CGA emulator TSR has to do is to read the CGA area, and do it's best to convert large 4-bit color pixels into small 1-bit pixels by dithering, and write to the active Hercules area.
The EGA has it's graphics mode frame buffer in the A0000h-AFFFFh area, for which the Hercules card does not provide the memory, so that alone makes it impossible to emulate EGA with Hercules. And because Hercules card by default is in monochrome text mode and has no way of mapping to EGA/VGA addresses, it can actually co-exist in a system with a EGA or VGA card.