Thanks James.  I worked out a solution, and it is in line with your comment.  Here are the specifics of what I did, in case anyone wants to do the same...

I declare two RPE data sources based on the RTC workitem schema (which are added in RPE using the following schema URL: https://myserver:9443/ccm/rpt/repository/workitem?metadata=schema)

The first data source I call "CCM Work Items", and it gets me the ID, summary, description, target iteration, and type of all the work items in my RTC project called "CCM_Sandbox".  The URL address I supply in the RPE document specification for this data source is:
 https://myserver:9443/ccm/rpt/repository/workitem?fields=workitem/workItem[projectArea/name='CCM_Sandbox']/(description|id|summary|target/*|type/*)

The second data source I call "CCM Links", and it gets me the sourceRef and targetRef of all the RTC links of type "implements requirements".  The URL address I supply in the document specification for this data source is:
https://myserver:9443/ccm/rpt/repository/workitem?fields=workitem/auditableLink[name=com.ibm.team.workitem.linktype.implementsRequirement]/(sourceRef/comment|targetRef/uri)

In the RPE template, I start by querying all the work items in "CCM Work Items".  I provide an external variable to let the user specify the iteration name they want to use to filter the work items, and so in the template I only handle those work items where workitem/workitem/target/name equals the value of that external variable.  For each work item found in this query, I query "CCM Links" for a link where the ID of the work item from "CCM Work Items" appears at the start of workitem/auditableLink/sourceRef/comment, followed by a colon.  You can perform that check with the following javascript snippet: if(comment.indexOf(id+":") == 0)

Finally, for each link that matches the above check, I dynamically configure an RRC data source called "RM Text" (based on the schema: https://myserver:9443/rm/publish/text?metadata=schema).  For the dynamic configuration URI of "RM Text", you need to use workitem/auditableLink/targetRef/uri from the "CCM Links" query.  However, that raw value is not in the correct format for an RRC data source.  To get it in the right format, you have to do the following javascript string replacement on workitem/auditableLink/targetRef/uri: uri.replace("/resources/", "/publish/text?resourceURI="), and use that newly updated string as the dynamic configuration URI.

Once you've hit this point, you are home free... "RM Text" is now the requirement that is implemented by the work item.  The approach I've described above is the most efficient way I've found to write the RPE script I was trying to write.

Regards,
Joe