Replacing the RAM would probably be straightforward - on the CGA its only 16K, so only twice as big as the font ROM. It's slightly trickier, considering the address lines are row and column multiplexed, and there are eight sockets to tap, and you have to be able to support reading and writing.

The CGA is dumb and re-fetches memory eight times per character row, so what you could do is tie in to HSYNC, so you could basically synthesize three new address lines from the row counter. That would give you a virtual 128K of video memory - a single character cell could then have 8 different sets of foreground and background colors. You could make some pretty impressive composite art with that!

I will likely do this for fun at some point, because I want to see "Never Gonna Give You Up" in the classic magenta & cyan palette, but this is not going to be something that anyone else will likely ever do as the RAM is soldered in - and nobody but Omega-level nerds are going to de-solder eight chips for a meme.