Scape pubmed for PMCID's and then use these to scape PMC for XML, convert to R objects which can be queried with get_articles_by_terms() + some new helper predicate functions. I am guessing that Pubmed frowns on this approach!
Download PMIDs from Entrez directly via a modified rentrez_search (Can you download full text XML from PMCOASS via entrez_fetch?), convert as above
Then we need to add functionality to fetch_in_chunks() to automate this...