It would be a terrible scenario if a G5 team had an undefeated season beating two top 25 teams, was ranked in the top 5 in both the AP and Coaches Poll yet had to beat two more G5 opponents (possibly ranked) just to play the number 1 seed in an 8 team playoff.
If no one in the G5 is ranked in the top 12, there is no reason to include them in the playoffs anyway and if they are ranked in the top 8, they shouldn't have to play extra games just to make the playoffs. If a school from the MWC or the AAC goes undefeated and isn't ranked in the top 8, conversation of a 2 team play-in game can be brought up.
The play-in game I would consider is the highest ranked G5 conference champion (if they aren't in the top 8) versus the 8th seeded P5 team at a pre-approved neutral site. If the 8th seeded P5 team beats the top ranked G5 champion the first three years, then the play in game would be dropped.