Add a 'values' parameter for datapoint search that hooks into the PSQL "@>" operator that allows querying by property, e.g. in the gltg db:

select * from datapoints where datapoints.data @> '{"site": "03336890"}' ;

I would implement this with:

/api/geostreams/datapoints?source=HRU&filter={"site":"03336890"} 

...or similar.