Even with a reprogrammed palette, the EGA spec allows 16 colors out of 64 choices. This is 1/4 of the colors of the 256 color space, but a judicious 'best match' algorithm and a port trapper TSR could maybe half-ass it on an EGA card. (EG, it traps the port write to set the VGA palette, parses the submitted write, then reprograms the EGA palette with a "best fit" approximation.

This might not work in all cases (like games that want to use palette reprogramming to do color cycling effects, fade in/out, etc...) but it should be mostly OK for 64 color VGA modes via a VESA driver.

There are no real extended EGA gfx for 640x480 + 800x600 with the normal EGA palette. The only thing is loading 16color VGA images and load the EGA.PAL to convert with "next best color".