Schema mapping doesn't apply to anything except modeled views. Therefore the only option here is to create a calculation view on top of these objects, apply the schema mapping to the calculation view and then grant access via the hdbrole to the calculation views instead.
Although, this concept of Schema Mapping is gone in SPS 11 with the new HDI approach. With HDI, all access must be local to the Container (Schema). Any cross container access can only be done via database Synonyms. We introduce the new design time objects hdbsynonym and hdbsynonymconfig for transporting such assignments. As this new concept relies upon the standard database concept of synonyms it is much more flexible than schema mapping since it can be accessed by roles, procedures, calculation views, SQL, etc.
See Page 81 of this document for more details on the new SPS 11 functionality:
http://help.sap.com/hana/SAP_HANA_Developer_Guide_for_SAP_HANA_XS_Advanced_Model_en.pdf