I can't know for sure but it look like there's a bug/oversight in /wp-admin/includes/media.php.
The media inserting pop-up iframe has 4 (or 3 depending of if there are any media attached to the post) tabs. Each tab have a form (I mean <form .. > ..</form>). Each form has
$form_action_url as form action. But only 2 of 4 tabs ("From Computer" and "From URL") has media_upload_form_url filter applied to $form_action_url variable.
I'm sorry if I bothered you in vain but to my mind it will be obvious if each form action will have these filter so this media inserting dialog pop-up can be utilized for plug-ins use.
For developers who also faced this bug:
Here is my solution overcome this bug without touching /wp-admin/includes/media.php.
Why calling the pop-up you can add a $_GET variable to request string and then use _wp_http_referer variable to track if the pop-up was opened with your code or with generic WP posing routine.
Of course it is not too pretty but I don't see other solutions unless this bug will not be fixed.