It seems it's impossible to customize the icon of an existing widget.
But depending on the app, we may be able to create a widget that launches an external application.
(Old) Source for Shazam:
Although not exactly what I was after, I have found a way I can get hold of them for now.
Stupidly, I didn't even think to have a play with desktop Maps. I followed the image URL for an icon when using directions on the desktop app which gave me a PNG with, what seems to be, all of the icons - maneuvers-2x.png
May be of use to anyone else interested