Hi Jeremy,
I hope you like links, because I'll send you a few:
Note that this schema does *not* contain a line that says: "additionalProperties": false
So practically, this means that additional properties are allowed, and that you can add additional fields to your dataset_description.json file.
However, please consider whether these fields may also be of interest to other people. If yes, consider opening an issue on the BIDS specification repository:
That way, we could potentially add new defined and standardized fields to share among the community.
Best,
Stefan