Hi John,
There is now where OADP and PCR comes into picture in regards to the Leave request application in ESS.
If you want the maternity leave to be applicable only for the female employees this you can restrict through the BADI - CL_PT_ARQ_REQ and include the explicit enhancement/validations in the method - IF_EX_PT_ABS_REQ~SIMULATE_VIA_BLOP .
Hope this solves the issue.
Regards,
Mithun k