Cool! I'll add a link on the resources page if you do end up putting it out there and drop a mention here to let me know.
So this .ans format can technically support additional colors on these systems, eh? (and arbitrary dimensions!) Note that RP already does .ans exporting, although it uses the traditional more limited format. (And the separate version of RP pre-configured for .ans editing also uses narrower text fonts by default, which are more likely to match the target ratio.)
I don't have any plans to open source REXPaint, but more versions and features will be released down the line.