The first goal of this is to help MM work better. My first point was the lobby dying as soon as people see crews of 45s. a 2v3 would change the mindset of the match from "Well, I don't want to get stomped." to "All right, let's take on the big guys." Since it is an option, the higher MMR team is choosing to do it or not, so no complaints there. I am sure some teams would rather go against three lower MMR ships with just their two than get a random third ship.
This is how I would see it working. Example given is 2v3. Another option would be available for 3v4. No 1v2.
1. Crew form with 2 ships.
2. Select "2v3" as an tickbox option like the other modes (or don't, see 4), Being on the 2v side would only be available to substantially high level players. I guess that would make this end game stuff.
3. Start MM. Get in a lobby. If you did not check "2v3" as an option and wait time is getting long, go to 4.
4. MM can't find an even match, but sees a possible 2v3. A pop-up comes up saying something like "It is taking longer than expected to find a balanced match. There is a 2v3 available. Include this in your search?"
5. Click yes and go to that match.
6. Optional feature. Instead of asking to include in the search, it could ask "Preview this match?" which would open up a new screen that would show you what the lobby would look like, and ask if you want to join. This might be the better option, since you would be placing your team at a one ship disadvantage.
Ingame tactics would be interesting. I see people have already dealt with situations like this, so it is not a new concept.