Enter the town menu via a script.

I wonder if it's possible to set some impossible coordinates for him, like -1000, -1000, so that he wouldn't be able to be seen at all, even with the map fully revealed...

Yes, you can 🙂

However, no testing has been done, and it might lead to some crashes. The developers probably didn't pay much attention to extreme cases: for example, when the special cursor mode with terrain recognition tooltip is enabled, moving the cursor outside the map leads to an immediate crash and window closure