Can you query the RCSB PDB API with a ligand and return the IDs of all entries in which it appears as a free ligand? If you're looking at the entry for a ligand, for example, adenosine, there's a link on the webpage to entries for which this entry exists as a free ligand. 
However, I want to access this via the API and can't seem to find docs/info on how to do that. 

I have been looking at the fields you can use to get custom reports.
I'm not sure about the RCSB, but you can get such a list from the PDBe REST API, as documented in the compounds section:

http://www.ebi.ac.uk/pdbe/api/pdb/compound/in_pdb/ADN

Alternatively, from an older service:

http://www.ebi.ac.uk/pdbe-srv/pdbechem/PDBEntry/download/ADN