It's an interesting requirement. :smileyhappy: I don't think QOS will help us for above requirement. Could you please try to create a custom vulnerability-profile with below mentioned parameter.
Set the value as 500
Then create a security policy (at the top) for Application=Youtube-base, Profile=Custom-vulnerability profile and Action=Block.
This configuration will block/alert all the http traffic where the http content header will be greater than or equal to the specified number in the customer signature.
It is currently not supported but as Vince said this is an interesting feature and i would encourage you contact your local SE so he can file a feature request on your behalf.
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