How can you specify an ARS control 'reason code' via SPML operation?

I supposed this request applies to any task executed through SPML request.  I am looking to provide our help desk an easy way to mass delete workstations matching a specific stale/idle day range.

Until an API is ready for primetime, I'm testing spml delete operation through simple PowerShell request.

This works - but I'd like to provide the ability to apply a reason code to these SPML operations ... 'Delete at 120+ days stale', 90+ etc .. 

Where do you specify the reason code in an SPML delete request so the value appears in the 'User Activity' section of the initiator's change history / or get-qarsoperation lookups.

$spmlhost = 'My ARS SPML Server'
$object = 'full DN of the object to delete'

$CLASS = "Active_x0020_Roles_x0020_SPML_x0020_"
$ARSOAP = New-WebServiceProxy -Uri "https://$($spmlhost)/ARServerSPML/SPMLProvider.asmx" -usedefaultcredential -Class $CLASS -erroraction stop
$ARSOAP.PreAuthenticate = $true
$Type = $ARSOAP.GetType().Namespace
$deleteRequest = New-Object -TypeName ($type + ".CdeleteRequest")
$CPSOID = New-Object -TypeName ($Type + ".CPSOID")
$CPSOID.ID = $object
$deleteRequest.psoID = $CPSOID