The medal "Beta Tester - Participated in the Beta" is locked for me as I did not take part in the beta tests. Does this mean that I cannot complete the game to 100%?
I suppose it depends on what you consider to be "100%" in the game.
Getting all of the achievements is what is required for the Xbox Dashboard to put a game in your "completed" list. If you're going by achievements, there is no achievement that requires you to get every medal (Seriously 3.0 specifically does not include this medal), and no specific achievement for this medal.