Browses on instrument and series are possible (if currently undocumented) based on a collection id:

https://beta.musicbrainz.org/ws/2/series?collection=b59f5ba9-40dd-4b08-b62b-dec922e7b2b9
https://beta.musicbrainz.org/ws/2/instrument?collection=5f90d4a4-de75-4e15-ab90-44d639634048

However, if JSON output is requested, these give "file error - main/500.tt: not found":

https://beta.musicbrainz.org/ws/2/series?collection=b59f5ba9-40dd-4b08-b62b-dec922e7b2b9&fmt=json
https://beta.musicbrainz.org/ws/2/instrument?collection=5f90d4a4-de75-4e15-ab90-44d639634048&fmt=json