I know that holding down shift disables that feature temporarily but it would be nice for some things such as custom offhand attributes to be shown correctly.

Here's a picture I took from an item on my server that alters your attack speed (+17.5%) and damage (-6%) while in the offhand.

It seems I can't do that on a bukkit server. I'm also not finding a way to use the command on singleplayer. I can however give you a vanilla style screenshot of the item stats right now, if it helps:

The main hand attributes both use operation 0 and the offhand attributes both use operation 2.

I've tried it. I can't use the command on my server (only in singleplayer) and downloading the world would take quite a lot of time. Copying the player data onto a singleplayer world was also unsuccessful unfortunately.