Is there a standard method for sending a message (or a private link to where the message can be retrieved) to an OpenID URL or URL in general..?
I would prefer a way to send a private link, but failing that, any kind of mechanism to send text to a (openID) URL will do..
The purpose is that the owner of the URL will then somehow be notified of the message/link..
You can't. At least, there isn't an easy way.
An OpenID is just an identifier like a userid that happens to be a unique URL. The protocol itself does not specify a way to send messages or emails to that identifier.
If an openID provider happens to be an email provider, you could perhaps figure out the email from the openid link. But that isn't a solution that will work across board.
If the provider supports it, you can use OpenID Attribute Exchange to get the email address of the user from the provider. Google supports it, but I don't know if other providers do so.