As I read things, I don’t think the backrub application can handle symmetry. You can try passing it a symmetry definition file, but I’m guessing it will ignore that option, and give you monomeric output, if it gives any output at all.
On the RosettaScripts end, BackrubDD is likely to mess up your symmetric pose (it fiddles with the foldtree).
I think the plain Backrub mover in RosettaScripts might work with symmetry, but you’ll want to explicitly set the residues you want to use, to avoid issues with autodetection glitches. I don’t think anyone’s tested it, though. I’d recommend trying it and seeing if it gives results you expect/like.
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