Hi David,
The current REST API of ALM 11 doesn't provide any interface for add/remove "Test Coverage" to a requimrent in Requirement module, and add/remove "Req Coverage" to a test case in Test Plan module. There is an ER for this that can be found here: http://support.openview.hp.com/selfsolve/document/LID/QCCR1J22908
Thanks,
Hristo