I think it's overkill for an MLT, but on the other hand, insanity and obsession is a prized trait, imo, so I say go for it.
Yes, those coolers have a very good thermal insulative value. Did you know you can also buy the foam by itself to make your own cooler? Just glass in the foam to whatever shape you want and voila! I thought that'd be great for for cask conditioning or lagering. Especially where people are using dorm-type fridges. Those things have poor insulation and, as a result, the electric bill on them is terrible. I'd be very tempted to use marine-grade super-high R value foam insulation for those kinds of purposes.
As for the cooler, same thing. I'd rather use it as a lagerator or something. But, if that doesn't work for you, and I doubt it'd be easy to convert a pre-made cooler to a lagerator, then I say go with your original plan and make it into the supreme ultimate MLT. There's nothing wrong with having the best insulative overkill MLT in the history of man... nothing wrong with that at all.... You should call it "My Precious".