I can one up you.
I have a co-worker whose husband went to Purdue...and is an ND football/IU basketball fan

I actually knew a girl at Purdue who was an ND football fan, but didn't really get into basketball at all. She grew up in Indianapolis and her dad was a big ND football fan (but didn't attend ND).
I don't know why she came to Purdue. She was an elementary education major. Seemed like IU would have been the perfect school for that, instead of hanging out with us nerds.
Does any team outside the preseason top 25 have any chance to win the NC? By "any", I mean something higher than 0.001%.
Probably only the top 12 or so have some realistic chance, maybe the next 13 have a chance of something like once in 20 years? Would be interesting to calculated based on experience, but we won't have that. Just making the top four would be possible, I think.
I think you'd have to find a team that was just outside the top 25 and had a bad previous year but still had a lot of talent on the roster. Probably in a weak conference / weak division, with a cakewalk to their CCG. Even then, they'd have to be 13-0 to have a chance which would probably take some lucky breaks, just to get to the CFP.
From there, you just have to hope that they have enough talent to compete and things fall their way. Beating two top teams isn't impossible, but it would be REALLY hard.
I'd call it greater than 0.001%, but not by much.
This would be one of those good times to look at the 20,000 simulated season type predictions and see how often it occurs.