I could see it being used to verify wall-ins, but this is an overkill solution to that problem. It may also locate some degenerate pathing cases. But it will still be a lot of work to get fully functional, even with a ton of the groundwork being done thanks to TSCMoo letting me base my implementation on the OpenBW one.
This is what it looks like now, it displays the path the selected units would take to the point under the cursor, and is continuously updated as the cursor moves.
They don't appear to take the shortest straight line path, but I believe the results accurately match what the game does.
This was from a previous version with predefined endpoints:
The extra sidetrip appears to match what SC does from in-game testing. I guess the marine likes to go sightseeing.
E: No idea why the images work when previewing the post, but do not work when viewing the final one, just click em, they work.