BIDS single participant data sharing - where do demographics go?

We are developing a pipeline to produce single-participant data in BIDS format. This will be relayed to a central site and merged (i.e., stacked) there.

What is the preferred place for demographic data in this scenario? Usually, this should be in a (multi-participant) participants.tsv/json file in the study root directory. However, in our scenario this is impractical for data merging.

Maybe check the BIDS extension proposal for phenotypic data.
It has a suggestion ho how to “segregate” this type of data per participant.

Note that this is not yet valid BIDS but this may give you pointers that can help. Feel free to chime in on the document if you have any feedback.