If it's not important enough to send a note via certified mail, then it's damn well not important enough to MITM my HTTP sessions.
That's how ISPs in Germany do it and it works fine. Or if it's urgent, they'll just call you.
No need to invent a new form of communication or do evil things with Squid.
This problem you describe is solved. Businesses all over the place manage to figure out how to communicate with their customers, but you want me to believe that Comcast is in some impossible situation where their only choice is to corrupt the data they're transmitting?
If you need to get in touch with your customers, then you can:
1. E-mail.
2. Call.
3. Send a letter.
4. Send a certified letter.
And if none of that works, disconnect the service since there's obviously nobody living there anymore. If the $4 for a certified letter breaks the bank, then add a clause to your service contract that says the customer has to pay it back if it ever gets to that point.
Not every problem needs to be solved with computers over the internet.
What ISPs don't provide an email account (usually something like username@isp-domain) with their service? I thought that was the standard way to do things, and it's what mine does to send me alerts about service outages and maintenance windows.