One interesting perspective is to use ListenBrainz APIs for lookup. LB already has to solve the issue of matching track metadata to a single recording.
Currently in the official API there is a metadata lookup endpoint, see Metadata — ListenBrainz 0.1.0 documentation . There are also some unofficial / experimental APIs. And the new mapper (MBID Mapper version 2.0 preview has finally arrived!) looked promising. Unlikely we’ll see this finished and deployed soon, but it looks like a great option for outside of LB as well.
@rob and I had been experimenting with using LB APIs for Picard as well. It was all just experimental, but results where promising for the use case of looking up single tracks. I hope to get back to this once we have Picard 3 out of the door 