I'm experimenting with xrandr's --scale option for this problem. For my external monitor with a native resolution of 2560x1600, I can run "xrandr --output DP3 --panning 5120x3200 --scale 2.0x2.0" to have it act as a 5120x3200 pixel display which is then downscaled to fit the screen. This makes the hidpi-rendered stuff appear at normal sizes on that screen.