Is it possible to GET the value of a custom attribute of a TER via REST API?

If I try with the below REST API:
https://localhost:9443/qm/service/com.ibm.rqm.integration.service.IIntegrationService/resources/RQM1/executionresult/urn:com.ibm.rqm:executionresult:63

(RQM1 is the project area and 63 is the TER id), I do see the custom attribute returned in XML:

<ns2:customAttributes><ns2:customAttribute required="false" type="SMALL_STRING"><ns2:identifier>TC_Result</ns2:identifier><ns2:name>TC Result</ns2:name><ns2:value>REST API</ns2:value></ns2:customAttribute></ns2:customAttributes>

Here, TC Result is the custom attribute name and its value is REST API

Yes, custom attributes are in the responses returned by the REST API.  What REST call are you using?  Does the execution result you are querying for have values populated in the custom attributes?