You could use the mob effect of 'invisibility' for a client-side UI element to denote invisibility, which wouldn't require Spout. I don't mean to use the mob effect itself, but just to tell the client that the effect is activated, which would display the 'Invisibility' effect while the inventory is opened.

Awesome! Thanks! :)

Tried contacting you on irc to demonstrate it D:

Ta-da

Oh, sorry, I've been at school all day :P