Development has informed that it is not possible to provide a fix as SAP does not provide such search criteria directly over the SAP tables. Identity Manager (1IM) is using BAPI (e.g.: RFC_READ_TABLE) to access SAP and so pre-filtering as a system filter cannot be implemented.
A workaround is to pre-define a custom schema type including specific filter and extend the schema with this schema type. A sample schema extension (HRP1000 table) is attached here for sample reference.