Is it possible to invert overlays? I'd like to place overlays on glass and make it match on both sides.

Open the properties window on the overlay and switch the U Start and U End values around.

Thank you!

For custom overlays, you might want to add the following parameter:

$nocull 1

That will make whatever texture to be visible on the back side as well, therefore, you can use it on glass and get the effect that you need.