I do not believe that you can do what you are asking for. Personally, I would use a TupleProcessor for this.Alternatively, (assuming, based on your example, that you have some mapping logic between your DCR and SP page), you can set an EA that contains the value of SP Page and then in your DD just use that EA value. No substitution and no tuple processor necessary. More than likely set that EA at WF time or onSave event.HTH!
Yea I did it with a hidden URL field in the DCR. I was just wondering if I was missing something in the docs.