The channels other than local NBC could be in a mix. And with an agreement with a broadcaster (perhaps a broadcaster who has agreed to carriage of three special feeds plus the regular channels, such as NBC) a mix channel shouldn't be a problem.
And there is the catch. They have to agree. I can see them allowing the soccer and basketball channels but as the big network is so important to them and to the affiliates, they probably nixed putting the NBC main network on.
Not sure what other major networks are on mixes for other sports. I know the CBS feed is on the masters but the masters is commercial free and actually owned by Augusta national not CBS.