Can you use MoveUpdate with a constant speed instead of "time" option?
Yes you can...
http://dkoontz.wordpress.com/2010/10/27/itween-visual-editor/
But yeah, that'll help you out, just drag and drop the main script on to a gameobject, then a GUI will be on it, and you can set the speed...
Only problem I've seen, is its really... snappy? Like, it kinda jumps to each position, but maybe it'll be different with you.
All the best!
MoveUpdate is, in essence, a real-time ease based on the model of "take half every step but never actually arrive" - meaning that it will always slow towards it's requested target. If you need linear (constant speed) movement you won't be able to get that out of iTween's "Update friendly" methods. I would suggested going for Lerp as you don't always need a jackhammer to hang a picture.