I don't think one more main menu option would clutter up the screen that much.  It would be just a "Hiergamenon" button, which would open the whole thing.  And if they want to make it even more user-friendly, they have the option of calling it "Help" and only use the Hiergamenon word once you choose the Help option.  This would help link the two words in gamers' minds without having to do much work.

I like this idea, there is so much content in the game now that having Hiergamenon on the front menu would be very nice.

Worthy of a bump.

There also needs to be easier access to the hiergamenon from every aspect of the game while in-game.

good call.

The game assets aren't loaded on the main menu.  We don't know if you are going to start a mod, scenario, load up a save game with older assets, etc.  So we can't show the hiergamenon for them there.

Civ did load the assets at the main menu which had its own advantages and disadvantages (you had to restart the game to change assets (such as to load Fall from Heaven).

Not saying either way is better or worse, but it isn't simply an issue of putting a button on the main menu, I would do that in a heartbeat.  It's changing the way the game initializes to support that change that makes it tough.