having a brain fart ATM. thought we had set this up prior but can't find notes. google and SW forums have a lot of posts for "send as" but not many with dirsync groups from AD. the closest seems to be http://community.spiceworks.com/topic/334324-give-a-group-send-as-permissions and its from may 2013.
before dirsync we did it the old PS way but I thought there was a simple adsiedit change that allowed this on the AD side to sync up in more recent version of dirsync this year.
beuller?