Is it possible at all to get a pic of it?
If I could see what is left of it I could possibly think of something, I thought of heating something up to make two holes in it to turn it with something like 2 picks, but if it is broken off how I'm thinking, that would be challenging to do without messing something up, like say you push the picks in to hard/too hot and go too deep, you do not want to pierce through the radiator.
If you do get it out then here is the link I put in the other thread for the part you need to replace it, it is part #18
I have heard of issues some can have when draining their coolant through that, but don't feel bad, I have always used it, and always will as it has never given me a problem, I put a little silicone on the o ring that is on it and I hand tighten it when done draining the radiator, so far I've never had a leak.
I only remove the lower radiator hose if I am totally flushing out the system, as it is easy to hook a shop vac to it and get most of the coolant out of there pretty quickly.
Given that it does not leak, and you can easily drain the coolant from the lower rad hose the next time you service it, it may just be best to leave it as is as Bill suggested.