How can I pull a UserValues value to use in a Routing Rule suppression logic?

If you create a class that extends Ens.Rule.FunctionSet, you can have a method that's selectable from the expression editor in either a DTL or a Routing Rule ...

Class User.Util.MetaData Extends Ens.Rule.FunctionSet
{
/// Retrieves the UserValue associated with key <var>pKey</var> from the message object supplied as
/// <var>pMsg</var> (normally <strong>source</strong> in a DTL or <strong>Document</strong> in
/// a Routing Rule) as a %String. Returns an empty string if the key is undefined.
ClassMethod UserValueGet(pMsg As EnsLib.HL7.Message, pKey As %String) As %String
{
    If pMsg.UserValues.IsDefined(pKey)
    {
        Return pMsg.UserValues.GetAt(pKey)
    }
    Return ""
}
}