Can a sample list have an attribute that is a sample list?

I'm trying to set up a gCNV "case" workflow where a set of samples are (effectively) evaluated against a panel of normals.  The panel is generated by a workflow on a sample set entity and its outputs become attributes of that sample set.  Then running the case workflow becomes complicated because it needs to reference multiple of these new attribute on the PoN sample set.  Can I add an attribute to the case sample set that refers to the PoN sample set so I don't have to manually copy any output paths?

One alternative solution is to add the PoN outputs as workspace attributes, but we foresee a use case where a workspace contains a cohort with multiple batches that requires multiple PoNs, in which case the workflow attribute solution would involve changing the config for every sample set.

The other solution some members of the team have used is to run in Cromwell outside of Terra with a script that puts the PoN output paths into the case input json.  Not particularly elegant either and we want to be able to get this into a user-friendly format for Terra.

Thanks,

Laura