When I embarked on DocStar for UK PS for 2018-19 first thing I did was ask about accessing other Epicor Services and there was a prototype REST dll available internally which you place DocStar side, with a json config file to determine the REST URIs and inputs/outputs. After much testing, and protyping, I was told this could be used on customer sites on proviso that at that time (and probably still) it wouldn’t be supported outside of Professional Services, i.e. an EpicCare ticket wouldn’t get you very far if using that dll! Also tested it successfully with SaaS deployments of ERP and DocStar.
It’s in at least 3 UK sites I know of!