Right now, even though we have both instrument and work entities, there's no machine-readable way to find works for specific instruments, you mostly just have to try searching for various instrument names and filter out the ones which are false positives, e.g. https://musicbrainz.org/search?query=%22for+piano%22&type=work&limit=25&method=advanced and https://musicbrainz.org/search?query=%22f%C3%BCr+klavier%22&type=work&limit=25&method=advanced so I think it would be good to have a relationship type for that.

IMSLP has categories for that, e.g. http://imslp.org/wiki/Category:Instrument_Composition_Lists
Wikidata has a property too - https://www.wikidata.org/wiki/Property:P870