In IC 5.5 or thereabouts, tongue bones were added to the G5 standard characters. I have played a little bit with that at the time. This is the procedure:
You will need 3DXchange to prepare your standard character.
Load your character, and then open the Expression Editor (option is on the right side).
Click the Custom tab:

Notice that you can define Custom motion groups (up to 6) with motions that are controlled by moving the mouse in different directions.
For each direction, you specify a target pose. In my screen capture I'm using the left mouse movement of Group 1 to move the tongue to the left (from our perspective). This pose is actually built from a number of poses:
- slightly open the jaw
- move the tongue outward
- rotate the tongue to the left
Once you are satisfied with the pose, you can retain it by clicking Set. Repeat this for other poses, such as moving the tongue to the right and sticking out the tongue.
Export the character back to iClone (or click Apply to iClone).
In iClone select the Facial Puppet and then select the Custom Group you just specified:

Note that iClone does not show what you selected (a bug!), but it's there. For some reason also, selecting a custom group causes the eyes to be selected as well...:unsure: When you deselect those, your group selection is no longer indicated, but it is there.
Now you can test the tongue motions by moving the mouse.
I hope this helps someone...:P