Well, MATE 1.20 (derived from GNOME 2.x, but now using GTK+3) is out and finally it supports HiDPI, so let's give it a try… Mid ‘2000s nostalgia rush incooooming!
Well.. There is HiDPI support indeed in #mate 1.20, but it's a bit hit and miss: many of the applets in the panel come with their host of glitches, and the window-manager decorations remain conspicuously unscaled. The systray in particular jitters in-place by itself and has good chances of producing seizures.
It was cute for a while to be using a desktop environment which takes the baton of the GNOME 2.x user experience and keeps it alive in 2018. If works fine if you don't have a HiDPI monitor. The #Mate team is doing an interesting job, but for a similar UX that does work better (with its own set of HiDPI glitches) I would rather use the Budgie desktop + modern GNOME applications.