I did pretty much what you you are trying to do and it was very easy to import the sprites files afterwards.
If you use a plain old rectangle for your placeholder you will need to add the events to trigger the Spriter animations after you have your scml ready. I would recommend that you have a rectangle anyway, and just pin your Spriter object to it, then hide the rectangle much like you stated above.
Alternately, you can animate your placeholder but you will need to change all references to the animations since Spriter objects have their own way of calling animations.
There are a few tutorials on BrashMonkey's site, but this forum post has pretty much all you need.
http://brashmonkey.com/forum/index.php? ... nstruct-2/
You have the right idea, and it's really not as difficult to implement as it may seem. Good luck.