Is there a way to define a single jboss-structure.xml file to use as a default for all SARs? Currently, it seems I have to define the same jboss-structure.xml file for all SARs.  It’s a lot of duplication I wish to avoid.

No.
But you can add your custom StructureDeployer which can do that.
http://java.dzone.com/articles/jboss-microcontainer-virtual-deployment-framework

I don’t know if this is applicable to jboss7, I am actually trying to do this in Jboss 6, but I can’t find a mailing list for 6.  If this is the wrong place to ask, I apologize, and if you could direct me to the correct place, that would be very helpful.

Yup, this is the *wrong* place -- for two reasons.
* this is *dev* not user ml
* as the ml name says, it's AS7 not 6

btw: there are forums as well ;-)