As far as I know, there’s currently no way to configure an instance of the subflow in the flow and the feature described here https://github.com/node-red/node-red/wiki/Design:-subflows-vNext is probably really far away (is anyone working on it?).
I would like to propose a simple change to expose the subflowName (id also to stay consistent with the latest change to expose id and name for the node) for all subflow nodes. With this simple change, we could do a subflow instance customization based on its name.
Let me know what do you think about it, I can send a PR to 0.19.x with it as I already have it working locally.