I believe that's already possible with TrueType now if the person doing the hinting is willing to make the effort. TrueType hints are restricted programs store

I believe that's already possible with TrueType now if the person doing the hinting is willing to make the effort. TrueType hints are restricted programs stored as bytecode. One instruction, MPPEM, requests the pixels per em. Another instruction, MPS, gets the current point size. There are also instructions to find pixel lengths across various axes. I'd have to think about the details but it should be possible to estimate the DPI and physical glyph size from those and then adjust the hinting accordingly.