According to the REST API guide the following endpoints can be accessed via HTTP GET to return a list of workflow rules, all workflow rules for an object, and a specific workflow rule, respectively:
/services/data/vXX.0/process/rules
/services/data/vXX.0/process/rules/SObjectType
/services/data/vXX.0/process/rules/sObjectType/WorkflowRecId
Upon giving this a go, I was getting an empty JSON object. Why won't this API return any data for metadata that appears to be there in the setup UI?