I'll look into it for the next update! The problem with these old formats is that there are often multiple interpretations/variants of each, and picking the "right" one is not always easy xD.
Also in this case I see support for features that wouldn't be accessible in REXPaint, for example assigning blinking characters and multiple fonts.
In any case, I'm not sure I could figure out how to implement them all correctly, and which would be supportable, but I'll see what I can do.
along with the ability to set the image width arbitrarily (e.g. 10 columns)
Note that this is already possible. You can set image dimensions to anything you want, and resize existing files etc. This particular feature can be see in the in-program help screen (and there's a button for it on the UI), but REXPaint also has a ton of other optional features that are only detailed in the manual.