The idea with the shadow or fog is that it would only hide the creature for the first few moments of the shot, putting the focus on the x-wing, and then the spider would emerge into view as the shot continues.
If any change was to be made, my vote would be to have the spider standing still for the first few moments, looking like a rooted plant, then become more animated as the shot progresses. (from McQuarries paintings, that's how I always imagined they'd have been introduced in the film)
Also, that's a pretty nifty idea in your mock-up up there, Ronster.