Glad the unicode and mirroring is coming in handy, at least! But yeah those features were only added late in development due to the broadened character support, and to be honest I likely would've added custom rectangle support as well at the time if anyone had suggested it.
Honestly I would patch in rectangle support right now if it was an isolated sort of thing, but proper support for that feature actually requires a change to the engine font loading, which in turn actually affects
Cogmind (based on the same engine) and causes unnecessary issues for anyone playing with custom font settings over there...
Ideally I need to allow the engine to be a little more flexible in its approach, so that REXPaint and Cogmind could have somewhat different font features. I'll put it on my list to look at sooner than 2.0, although Cogmind's next release has a deadline now and there's a lot to do before then, so it could still be a while before I can get to this since I know it will take a while to implement.
Which brings me to... where's the best place to put my donation towards the future development of this application!
Some people do direct PayPal donations via the website, and there's also an itch.io page for REXPaint which a few people use