People DO stir their mash, in non-airtight mash tuns. I'm not sure where you got the idea that O2 was such a gigantic problem during the mash. Of course I've heard people worry about HSA, but I don't think I've ever seen anyone that concerned about it during the mash. Have you ever seen the pictures of how they mash in commercial breweries? Big vats with big stirrers? I've never seen any evidence that their containers are airtight or that they purge oxygen from them, despite the fact that big breweries are the ones that actually DO have to worry about HSA due to the giant amounts of wort they pump around. Of course, I'm no expert on commercial brewing processes so it's possible I missed something.
I think if you've got the cash and initiative to even think about going that crazy with a nitrogen-purged sealed mash tun, you'd be much better off applying your ingenuity to some other part of the brewing process, on a problem that could actually benefit from a fix. Over-the-top DIY projects are one of my favorite parts of brewing, and of belonging to a brewing forum - but it's a lot better when they're really useful as well