Yeah the filter is added overhead. Then again, if we don't implement a filter, we will need the admin tool to run during upgrade, then again once the admin finishes putting together their whitelist.
The filter would work nicely for existing content, allowing the admin to locate items that don't work and gradually add them to the whitelist, without having to run the tool each time.
Is my understanding correct that a filter would miss URL resources set to display as embedded content? It seems to me that the external URL is not exposed to a filter in that case.
A filter could modify the embedded url, if it was programmed to do so. But it would not be able to change the url in the URL resource's settings. Only the admin tool could achieve that.
My attempt to write such a filter succeeded in influencing the intro contents of a URL resource -- so I know it is working -- but did not affect the value of the URL itself that was used to build the iframe markup. Looking at /mod/url/locallib.php I can see where some of the settings are being passed through filtering, but I can't see where $exteurl gets filtered. Is there some way, from within the filter (without hacking mod_url) to invoke a filter on that value?
I've tried the filter you proposed in connection with MDL-46296 and while it will modify an iframe src that is added to a textarea, it does not modify the "External URL" value for an embedded URL resource. I know that it won't change the setting, but it seems not to filter it, either.
In that case, perhaps a tool like enovation's would help? It could modify the external tool URL in the database.