One of the goals with this proposal is to create sort of a "funnel" of involvement by the teams. The initial stage represents zero team involvement; anyone can start a repo. But to move onto the next stage, someone from the team has to sign off on it: that's your champion. But it only requires one team member. Later stages require the entire team.
So it's not so much "speak for itself" as it is making sure that, as things progress, they fit into the team's goals, while also not bogging every single step of the way down in requiring all team members to sign off immediately.
As far as finding a champion goes, I imagine this is a good use of the team email aliases; once you think you're ready to move from spitballing to designing, you email the team and ask for a champion. If nobody on the team wants to sign off, that's a good sign that this either doesn't fit into the roadmap, needs to wait until someone's time is freed up (which isn't inherently a bad thing; it's part of prioritization), or something else. Ideally this is actual feedback given to the RFC author.
That said, that's just what I'm imagining; you're right that this is an important aspect of the whole thing, and so we should have a good story sorted out here.