I agree on the barrier comment. Not much chemistry can occur in a dried glue plane that is itself sealed from moisture on both sides. If you want to be totally acid free, try Lamin-All from Drytac, which is just like Miracle Muck but "pH Neutral." I think sodium bicarbonate may be involved.
I have to guess that a bare canvas substrate would glue a lot more securely than a coated one.
And BTW, most glossy canvases are fairly acidic in their own right, although the labels on the latest boxes of Epson's Exhibition Canvas Gloss now state that it is "Acid Free."