and then couldn't you just modify/override the Maxis EA TEST and interaction BHAVs.
Indeed, but the objective here is to affect the original tree without actually overriding a PersonGlobal BHAV or TTAB in the process. It really boils down to procedurally overriding the TEST conditions (and apparently the non-adult advertisement). Keep in mind that some interactions (not just PersonGlobals) share a common test BHAV (although I think these are extremely rare) and some don't even supply a guardian test BHAV at all you could even override.
Using your vamprocillin example, it's clear how you can "add" to the pie menu as you've done. What's not clear is how you can "subtract" from it. What if you wanted (for some stupid reason) to disallow adults from using the vamprocillin? I see no clear alternative but to override
Interaction - Use Vampire Cure - TEST to always return FALSE and then replace it with the version supplied via your social plug-in or (god-forbid) you override the PersonGlobals TTAB and untick all the flags *gaugh* so you now have the very worst of both worlds: a global override of PersonGlobals and broken advertisement to boot (not that anyone really cares about the latter in this particular example).
If you don't mind dinking around with Lua, I think there's a function for that:
OverrideTreeTable(objectId, value1, value2, bool)
Curious. Is there an example of its usage anywhere?