From The Athletic (everyone here should already have a sub to that site, I would expect? If not, you should).
Anyway, if you don't, it does list games with 4 p12 teams including the following:
#20 - 2001 Nebraska-Colorado
Granted, Colorado wasn't in the p12 at the time, but.... Anyway, what's interesting is that Colorado steamrolled the Huskers in that one (63-28), ushering the end of the Nebraska reign (which continues to this day....right, Frosty?....LOL) Footnote: Oregon would go on to crush Colorado in the Fiesta Bowl that year 38-16...with some arguing it should have been Oregon vs Miami in the 2002 BCS Champ game instead of Nebraska.
#11 - 2005 USC-Notre Dame
Significant, not so much because of the then-illegal "Bush Push", but because of Charlie Weis' 10-year contract that came from "almost" beating USC, paving the way for future guaranteed deals and giant buyouts.
#9 - 2009 USC-Oregon
The 47-20 drubbing of USC now known among Oregon faithful as "Fright Night" (it happened on Halloween at Autzen) signified the end of Carroll's reign and the beginning of Oregon's dominance in the conference, as 2 months later he would be coach of the Seahawks, and a year later Oregon would play for the national title.
#3 - Texas-USC Rose Bowl after the 2005 season
The 41-38 Longhorns win over the Trojans was the most thrilling of the BCS era and possibly the most thrilling of any era.
To me, the significance of the 2000 Nebraska-CU game was what didn't happen afterward. They (I don't know who "they" is) didn't say "No, if you don't even qualify for your conference championship game you can't be eligible for the national championship game." I'm pretty sure that was the first time this happened but the SEC has gotten several teams in this way.
And I think the greater significance of the 2009 SC-OU game was the shift of the conference balance of power back to the North, even though there weren't divisions yet. The North (Washington, Oregon, and Oregon State) held the power starting with UW's 34-0 blowout of SC in 1990. SC brought it back to the South in the game at Autzen in 2002 and it remained in the South -- well, with SC -- till that 2009 game.