I think I've found a resolution, but here's the process that I went through, and my resolution - either for future reference for others in the same position, or for someone to correct me with a better way to achieve this.
In the sub-workflow, the transition from the incoming jump point, does not have a table or a schema. This sort of makes sense, because the Jump in itself doesn't have a schema defined, and the sub-workflow can't know where it's going to be called from.

On the "Suppression Check" split, if I try to use the Temporary schema from the Jump, I get the following error, which again is not unexpected, because the Jump doesn't have a defined schema.

But I can use the nms recipient schema, and build my filtering query successfully:

Even though the Suppression split is using Recipients for the Target and Filtering dimensions, the output transition still shows no schema:

And of course this then means that there is no schema defined for the Data extraction step:

So what I've done is create a Query on the recipient table as well as an enrichment to get the linked attributes that I'm after. This results in me being presented with a list of available fields in my data extraction format:

And finally, put the workflow back to how it should be, and disable the Query:
