Will Notre Dame join the Big XII just in football...
No
...or will it be in all sports?
No
Notre Dame is not joining the Big 12 conference. I'm not convinced that Notre Dame will join any existing conference. The big 10 has the best chance of reeling in the Domers, but ND has repeatedly told them no. IMO, the only way Notre Dame joins a conference is if they build a new one from scratch.
In the last 20 years, they've played USC, Navy and Purdue 20 times, Michigan St and Stanford 18 times, Boston College 17 times, Michigan 16 times and Pitt 14 times. Those are their most frequent opponents and you can bet that they'll want to keep all of those series alive. If they're in a conference that plays 8 or 9 conference games, they'll only have 3 or 4 non-con slots available. How do you keep 8 long-time rivals with only 3 or 4 slots available?
One answer is to join the big 10 so that Purdue, Mich State and Michigan are conference foes, schedule USC and Navy every year, and set up a rotation with Stanford, Pitt and BC. The drawback to this is that series with Stanford, BC and Pitt suffer because they are only played occasionally.
Another answer is to form a new 8-team league that includes some of their long-time rivals, such as BC, Pitt and Navy. If they're in a conference with 8 teams, that means they'll play 7 conference games and 5 non-con games. That gives them the flexibility to keep schools like USC, Purdue, Mich St, Mich and Stanford on their schedule pretty much every year. I could easily see ND/BC/Pitt/Navy forming the eastern half of an 8-team league, with UT/TT/OU/OSU as the western half. This would easily be a BCS league so the winner has an AQ bid. TV networks would be lining up to sign a Tier 1 contract. The conference could eschew a Tier 2 contract completely and just let the schools form their own network (or join together with some of the other schools to form a joint network) for the Tier 3 rights. UT and ND would obviously have their own. OU may also. OSU/TT/Pitt/BC/Navy could probably join together to form a joint network or each could do something like K-State and have an online "network".
Either way, I just don't see how the Big 12 factors into ND's plan.