xrandr is perfectly sufficient for this, although GTK for whatever reason chooses to break it (not just not implement it; proactively break it). Xcb picks it up just fine, as does Qt. I'm pretty sure Fltk does now too though I haven't tried it in a while.
No, xrandr isn't sufficient. It still doesn't have scaling information, only size information. Changing the reported physical size of the monitor is a bad idea as it can break other things.
X11 simply isn't built to do this. If you want this to work, then the last 35 years of clients don't do the right thing and still would need to be changed to use a new extension that behaves more like Wayland does it.
