Are there any ways to do some sort of upscaling for HiDPI displays? I’ve always wondered what MacOS 8 or 9 would feel like with 2x retina style rendering.
I'm not sure what that would even mean. Most of the UI was drawn from bitmaps - there isn't any higher resolution internal representation of e.g. a window border or a radio button.
What it would mean is something like hq2x, 2xSaI or similar. These algorithms are popularly for emulation of 8 and 16 bit era games consoles. The example image on this Wikipedia page is illustrative of what's possible with a deterministic algorithm:
https://en.wikipedia.org/wiki/Hqx_(algorithm)
Imagine what could be possible with a well trained neural network, with knowledge of every icon, every element, and every font glyph at every bitmap size.