Session for Multi-Page Flow and complex objects

I have a multi-step flow where the primary model has a number of
"has_many" relationships. I've read of people storing their Model in
the session for multi-step forms, but for some reason I feel dirty
when I do this.

For my example, I have a slideshow:

  + has_many images
  + has_many invites
  + has_one

Invites is a simple mapping table which associates users to the
slideshow. I also want to store the raw cover image data in the
session until the slideshow has been created.

Is storing this in the session advisable? I'm using an active record
store session with MySQL, should I worry about overrunning the
"longtext" data field? What are other solutions that I could use?