You know, if you have different internal and external DNS zones for your domain, you might be able to use a generic block page with a link to different sets of instructions for internal and guest users. For example, "Visit webblocks.mycompany.com for details" might be the URL in the block message. Assuming your guest networks don't use your internal DNS servers, this could point to a public page which includes info about what content is blocked, the acceptable use policy, etc. On your internal DNS server, that record could point to an internal web server showing a page with instructions on how to contact support. This is definitely just a workaround, and only works if your DNS is setup as described, but I thought I might mention it anyway.
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