I have never really found plain text in the arcade roms. AFAIK each game has a translation table you have to figure out first, to tell you what hex value equals what letter. But, I imagine it could potentially be more complicated than that because what if they didn't store text in variables with as many bits as we normally would on the computer since they wouldn't need that many. A good example is color pallets on SNES. A pallet is a collection of colors, each color is represented by three 5-bit numbers. That gets pretty annoying to find in a file because something like a hex-editor usually breaks the file down into 1 byte (or 8bit) chunks at the least.

I usually started editting an arcade rom by pasting big chunks of 0's into the file at different places. I would put in a chunk, play for a while to see what the effect was, document it and try another area. I have only used this though to screw around with graphics in the game. The graphics make up so much of the rom file that you are much more likely to hit that versus the code.

Is there a way anyone knows to see the commands as they execute in any of the arcade emus? Maybe some way to see the individual commands that load stuff into memory so you can track it back to figure out what goes where?