You could then iterate through the returned classes and check to see which ones are marked "system" by opening up each definition. I seem to vaguely remember a way to do this with a query but I can't find it now.

Programmatically you can use the SubclassOf() query in the %Dictionary.ClassDefinitionQuery class. Pass in %Persistent and the flag for non-system classes.

So if I am calling this service from an Ensemble operation that looks something like this:

Method getPatient(pRequest As RESTDemo.REST.ClaimRequest, Output pResponse As EnsLib.REST.GenericMessage) As %Status
{
  try {
    // Prepare and log the call
    // Append the claim to the URL
    Set tURL=..Adapter.URL_"/claims/"_pRequest.claimBlob    
       
    // Execute the call
    set tSC=..Adapter.GetURL(tURL,.tHttpResponse)
       
    #; Suppress HTTP status errors and just pass back the status, headers and body
    If $$$ISERR(tSC)&&'$$$StatusEquals(tSC,$$$EnsErrHTTPStatus) { Quit }
       
    // Return the response      
    Set tSC=$$$OK
    If $IsObject(tHttpResponse.Data) {
      Set tStream=tHttpResponse.Data
    } Else {
      Set tStream=##class(%GlobalBinaryStream).%New()
      Set tSC=tStream.Write(tHttpResponse.Data)  Quit:$$$ISERR(tSC)
    }
    Set pResponse=##class(EnsLib.REST.GenericMessage).%New(tStream,,tHttpResponse)
    Do pResponse.HTTPHeaders.SetAt(tHttpResponse.StatusLine,"StatusLine")
  } catch {
    Set tSC=$$$SystemError
  }
  Quit tSC
}

Would I just call $system.Encryption.Base64Encode() on the claimBlob property?