The value from this operation is not persisted in the host xml file, nor is it reflected in a read-resource response:

/host=slave/core-service=discovery-options/static-discovery=primary:write-attribute(name=port,value=9999)

It also doesn't put the HC in reload-required state.

This works:

/host=slave/core-service=discovery-options:write-attribute(name=options,value=[{static-discovery={name=primary,protocol="${jboss.domain.master.protocol:remote+http}",host="${jboss.domain.master.address}",port=9999}}]

The /host=slave/core-service=discovery-options/static-discovery=* resources are meant to be a kind of alias for elements in that 'options' list to make it easier to manipulate the details. But that linkage has a bug. (Probably several.)