Yeah, I see the problems it could have, but it was just a fun lil idea I was thinking of. I could easily forget the Flatter/Swagger and equip moves such as Wrap or block to keep them in, but again that would still be taken down by the opponent switching out.
A way I could see this work, however, is by doing such a tactic as baton passing +Speed onto Smeargle, potentially making him outspeed all the opponent's (depending on what the opponent is carrying, one might need to go to +6 speed) to get off the combos such as Flatter or Swagger, and then commence transforming, I would lose the speed boost (not sure), but if done right, perhaps it would be worth it?
Pairing it with a Ninjask with Agility, Protect, and Baton pass (as well as a focus sash) could easily prepare my Smeargle with at least a +3