I made a whole thread discussing game mechanics that started with this idea: http://www.gamesas.com/topic/1338190-activities-that-freeze-time-and-those-that-dont/page__fromsearch__1

The idea would be, to briefly sum up the idea from my OP in the other thread, that only the System menu would pause time, to allow you to save or load a game or pause the game so you can go pee or grab a Coke without coming back to find your character dead. But if you wanted to do anything in the game, like read a book, check your map, drink a potion, change armor or weapons, and so forth, time would still be running. Think of it as the difference between Oblivion's conversation system, where time froze around you and the person you were talking to, and Skyrim's, where time marches on while you're chit-chatting with what's-her-name about how she doesn't claim to be the best blacksmith in Whiterun.

The advantages generally revolve around tactical realism:

• No more spamming potions in combat. Better have one hotkeyed if you really think you'll need it.
  
• No more wearing a whole set of armor or clothing enchanted with sneak or muffle enchantments, and then freezing time to swap out for armor just before that bandit's axe cleaves your skull.
  
• You can still use a bow for ranged combat and then swap for a melee weapon if the target closes, but you'll have to think ahead of time and have it hotkeyed or at least on a short list of favorites for quick access - and you won't be able to stick with the bow until the last second then count on an instant swap. Planning ahead becomes crucial.
  
• You need to think about your visibility in some territory. Want to stop and check your map? That's fine, but think about where you are. If you're on the road near Whiterun you might well be okay to just stop and peek at the map. If you're out in more dangerous territory, maybe you should check your surroundings first, find a little area where you're unlikely to have a wandering baddie get LOS on you,and then crouch into sneak mode for good measure while you're stopped.
  
• Same goes for reading books. Just found a book in a dungeon? Maybe you need to wait to read it until you get out, or at least make sure there's no bad guys present when you do so.
  
• Having a follower or two to watch your back while distracted wouldn't hurt either.

In general, the idea is that it prevents gimmick tactics like potion-spamming and last-second wardrobe and weapon changes, and it forces a tactical mindset on the player. I couldn't find any such mod on the Nexus - searching for "menu" brings up ten pages largely filled with mods that alter the Favorites menu and the Main Menu - and hell, I don't even know if this is possible. Maybe they hard-coded too much into the behavior of the menu system, and they unalterably freeze time when opened.

Still, it would be interesting to know if such a thing is technically possible. With all the people lusting after hardcoe mods, I would think this idea might spark some interest.

It won't be possible to get any of the "gamemode" logic to run when in "menumode" but one could probably fake the passage of time by manually incrementing the global variables that track the number of game hours/days passed.

Imp of the Perverse did this for his Complex Needs mod for FNV, so it should be possible. As for the actual game continuing in menumode, I doubt there's a way to do that. Also I'm pretty sure I couldn't play with something like that, Skyrim has way too few hotkeys to effectively cover all my needs.

Hmm. And I assume this menumode is simply hard-coded in, no way to work around it like SkyUI did the assets and look of the vanilla UI?

Yes.

Would it be possible to create some sort of new, alternative menu system so that the game wouldn't enter menumode at all?

And have it triggered with the same keypress that would normally trigger the regular menu system? I started to wonder that myself. Hell, what if the regular menu system's assets and functionality could simply be duplicated without actually entering the "official" menu system that freezes the game?

Since there doesn't seem to be a mod that does this, I've also got a thread in the CK forum asking if this is even technically possible.