Even though Delaware is on the fringes of CUSA territory, it is unlikely they will end up in CUSA.
If you go look at the UD forum, all the fan chatter is about joining the MAC
http://www.gohens.net/boards/viewtopic.php?t=29746
If CUSA adds, yes it would be from nearby schools like SFA, UCA, and Tarleton, though it looks like they want to "hold" for a while.
It is actually the MAC that appears open for business, not CUSA.