OpenRA

You don't seem to be able to do this. This could be useful if someone has disconnected.

Or you may be able to help someone by providing them with a new MCV for example. I had a team mate who sold their own MCV early in the game (gg), and they really needed another one of course...

This is more of a question than an answer..

Couldn't you become enemies for a few seconds, and then re-ally? The diplomacy tab should allow you this, if I'm correct..

This is how I used to do it with friends back with TS and RA2. >_>;

You need to allow diplomacy changes in the lobby settings, but then the proposed method should work.