Prequels aren't really an important piece of story content. It's just a beta (not much content); they could summarize it as part of the City's history in a Guardian briefing, or in terminal-type things in the Tower, or a million other ways. Or just don't summarize it; the main game would start a separate story that doesn't rely on the specific events of the beta, just makes connections that only people who played the beta notice.

I was thinking it could be talked about and show to Guardians at the beginning of the game.

Well, that's what us PS owners got with Mass Effect 2 so I guess that could work haha