Symmetry restraints


I wonder is there a convenient way to implement symmetry restraints (e.g. for multi-chain proteins) in GROMACS?
I understand that the actual symmetry groups are probably not supported, but I guess it should be possible at least to restrain the in-molecule interatomic distances and/or dihedral angles to their values in an equivalent molecule (like MODELLER does or to their averaged values (similar to what was done here with NAMD
The latter option seems to be possible using ensemble averaging, but some examples would be very useful!

Thank you