HBO really could do the same thing that they are doing with Morales and Pacquiao by having Zab and Floyd fight different guys on the same night in order to build up anticipation for a potential showdown between the two of them.
Hell, they did it with Hopkins and Taylor too, its not exactly unprecedented.
no it wouldnt neither one should ever be undercards.
for one thing its disrespectful to great fighters. and the main thin is it would be 2 fights in one day intead of saving one for the a week with no boxing on.
Zab's last fight was on an undercard, in fact, he hasn't been the main event on a major card since the Tszyu fight (maybe he was co-main event on the Klitschko - Brewster card, I forget) - the undercard is where Zab should stay until he makes another good fight. There's also no reason why Zab can't be a co feature on a Mayweather card in order to set up a later showdown - seems to happen all the time now