Unfortunately, all functions that require a target avi key default to the owner's key when the item is worn as an attachment. So you can't call the status of another person's system through the API (to find out what area can be licked), or set them as a lick target.
Edit to add: This is to avoid griefing and unwanted effects.
The other issue is that, even with yourself set as the target to lick, you can't completely bypass the lick area selection menu, there's no support for this in the current API, so although you can open the lick menu automatically (but only with yourself pre-selected as target) you still have to select which area to lick on yourself.
I've been able to somewhat automate it on a hud for licking myself, wiping my face and swallowing, but that limitation is still there.
The hud you mentioned doesn't actually lick or swallow anything, it simply removes it. This is due to those same and other limitations.