Since "/console unitHighlights" was only later released in the API in 2.4, I was unsuccessful so far to enable this for 1.12.1.
I assume it was in closed-mode previously.
Either way, I'm looking for someone who can help me out on this. I'm playing on Nostalrius Begins and would like to see this working... I'm also willing to pay if necessary because I can imagine that this can consume a lot of time.
But maybe doing this is impossible for Nostalrius Begins because of its strong security measures.
Sources
Patch 2.4.0/API changes
Download Patch 2.3.3 to 2.4.0 (US/Aus)
My thread on the official Nostalrius forum