There's a big difference between saying to the planner, "Use plan X" vs "Here's some information describing the data supporting choosing plan X intelligently". The latter allows for better plans in the face of varied/changing data, integrates with the planner in natural way, and encourages users to understand how the planner works.
+1
I was thinking of varying the 'weight' of a user defined plan by an fixed experimental factor to tell the planner to give higher/lower preference to this plan, but after your idea above, I think Stephen's point of introducing a GUC for the factor is the only way possible and I agree with him on the point that eventually the user will figure out a way to force usage of his plan using the GUC.