On my US keyboard, those shortcuts are ctl-alt, rather than alt-gr
It's effectively the same, Windows maps ctrl-alt to alt-gr, as original AT keyboards didn't have an alt-gr key.

Don't know if they're considered "Windows" standards at this point, but it's possible
Well not really, Windows standards are those keys supported by Windows. Word does a thousand other configurable little funny shortcuts and autoreplaces not supported by any other app. (Not all of them very clever, natch, but at least they're easily disabled.)

The US International keyboard layout does give © and ® on alt-gr+c/r, but not the other shortcuts (eg. ™ overrides þ). One or more of these keys also clashes with several standard Eastern European keyboards.

Losing those conventions would likely result in a lot of noise from users who're accustomed to them.
Well an option would be best of course, but Ctrl-Alt shortcuts are an acknowledged Wrong Thing (see eg. http://blogs.msdn.com/oldnewthing/ar...29/101121.aspx ). There should be plenty of alternative key combinations available.