ConceptMaps are not exposed by the restws and should be. Create resources and controllers for it in the webservices.rest module similar to other resources like ConceptDatatype, ConceptClass, EncounterType, etc.
This requires adding extra resources and controllers for ConceptMapType, ConceptReferenceTerm, ConceptSource and ConceptReferenceTermMap, see the sub tasks for this ticket
It would also be good (in addition but at least) to have a concept search that exposed ConceptService.getConceptByMapping. This could be done by using the q= parameter for <code> and adding a source= parameter for source name/uuid.
As much as possible should be taken so that this works for pre-1.9 mappings and new 1.9+ ones as well.