It's a total PITA, but it can be done as long as you are ok with full screen. I use it to run PixInsight on FreeBSD and view it on macOS: https://xw.is/wiki/HiDPI_XQuartz

/edit: oh, and yeah, if you patch xrandr not to return, I suspect that the resolution change will persist on switching between macOS and the unix desktop, though I haven't tested that yet.