For what it's worth, I did some actual research.
Cindy Mizelle, one of the LL's is a touring member of Steely Dan for 2015 -
http://www.steelydan.com/2kband.html
Their Schedule is located here
http://www.steelydan.com/tour15.html
It looks like on July 3 & 4th Steely Dan doesn't have any shows. So, unless she's committed to something else, she appears to be free for SPAC. The other two are Tawatha Agee and Sharon Bryant. I have not looked for what else they have going on this summer, but a little google searching should surface something.
The good news /bad news, depending on your point of view, for us Alpine folks, Steely Dan has a show on July 25th (Highland Park, IL) so depending on which band gets priority in having her (assume Steely Dan since they have her listed as a member of the tour),
Cindy Mizelle looks to be unavailable for Alpine Night 1!