Can notes on line items be updated on checkout?
I use ajax autosave to achieve this, and ajax update the cart in the background (for notes, but also pretty much everything else).

I essentially use the recipe from:
from https://github.com/geniuscarrier/autoSave/

In the callback function that is called after the autosave event, I post the to the update cart controller.  The JSON object that comes back from an ajax request is quite complete and let's your update your cart page / totals etc from there.

Personally I hate non dynamic carts with update buttons, so I have ajaxed all cart operations, including qty changes, removals, line item and order notes etc.