I don't know about anyone else, but my definition of a "cover band" is, generally speaking, any time a different lead singer is performing a well-known song. So if the new Mac sings Don't Dream It's Over, with Neil Finn singing, they're not a cover band; if they perform some Tom Petty songs with Finn (or anyone else) singing, then they're a cover band. Or when they sing Lindsey's songs = cover band. When they perform Stevie and Chris's songs? Not a cover band. Things get complicated on tunes like The Chain or Don't Stop, though!
So, I'd love to see NuMac perform a few Finn tunes, like I Got You or Message to My Girl. Seems pretty unlikely, though.