MTR
I tried a few ideas I had, but didn't get very far...
see this thread from 6 months ago where I made a sample that did something similar (but not outlining the total area):
https://www.construct.net/en/forum/construct-2/how-do-i-18/check-mouse-sprites-139913?kws=france.capx
I thought you might be able to use scale to make copies of the occupied map pieces, tint them to the border colour you wanted and move them under the real pieces so just the over-all outer edges were visible, but because of the irregular shapes, scale did not produce an even outline. So, that means you will probably have to make two copies of each map piece - one without a border, and one with a border. And then have two layers of pieces - with the pieces with a border under the ones without. The main problem is what happens when two players' map pieces are side by side - which way does the border extend? if the border does not extend past the edge of the map piece, then you will need border pieces for each edge of each piece and work out what to show.
you could also try putting image points all around the map pieces and use that to calculate where to draw a border, but that wont be easy either...