1.6 PTS latest update includes basic inventory API functionality.

[TimeBomb]

Source:
http://forums.riftgame.com/beta-addo...ml#post3315754

* Added Inspect.Item.List and Utility.Item.Slot.*. This gives the ability to inspect item IDs in the player's inventory and other similar inventory-ish areas. This API is highly incomplete and may change without warning.

Hopefully this means we will also be able to use the items.
If that is so, an addon that creates a custom one-bag should be possible. The problem is it would be a custom UI.
There has been no mention about being able to access/edit the preexisting UI; if that is added, then that would make the custom one-bag addon even better. Cross your fingers.