But even if I'm not readying an action, I'm treated as if I am readying one against any creature that charges me. So, it doesn't cost me an action, and it doesn't "deactivate" just because someone already charged me, because it never cost me anything in the first place, and it is an "always active" ability of the magic item.
I could see not allowing it as a DM, sure, but as for how it is written, I think you'd at least get an attack against anyone that charged you, no matter how many of them there were. I can see how you'd argue that you can't also actually ready an action, though.