I've been using the DigiTech XP300 Space Station / Synth Swell effect on mellotron output. (Built into the Expression Factory pedal). It will do one octave up or down and 4th and 5th interval up or down. Most of these sound very good. The octave down makes everything sound like T-Dream. The octave up sounds a bit fake with string voices but good with IM-Flute.
The one big problem with this effects pedal is that it has a thick space echo running with ALL of the XP300 settings. That's right... No way to adjust the echo tempo, decay or volume. I guess if the echo tempo doesn't fit with the song you are playing... You are just screwed.
Clay
Rover's first appearance:
http://www.youtube.com/watch?v=I6Ffr1U7KMY