Is it possible to make a custom texture for env_instructor_hint?

According to the VDC, it seems like it's possible. Have a look at "portal2/scripts/mod_textures.txt" and "portal2/scripts/instructor_textures.txt", you could probably modify those to add an icon. If you can, try overriding using dlc1/2 or even through BSP packing, it might work - regular scripts do, so hopefully this does too.

The file format suggests you can make essentially a spritesheet and select a bunch of icons from that, which might make packaging files a little easier.

ugh, it had to be scripts. I suck at scripts. I can cram this into the workshop, right?