Facebook OAuth: custom callback_uri parameters

I figured out the answer; rather than adding additional parameters to the redirect URL, you can add a state parameter to the request to https://www.facebook.com/dialog/oauth:

https://www.facebook.com/dialog/oauth
    ?client_id=MY_CLIENT_ID
    &scope=MY_SCOPE
    &redirect_uri=http%3A%2F%2Fwww.mysite.com%2Foauth_callback%3Ffoo%3Dbar
    &state=6234

That state parameter is then passed to the callback URL.