And mail pieces don't produce the potentially rather widespread indignation that traffic injection does. Granted, I don't see the harm in it that a lot of people here do. Unencrypted traffic is unencrypted traffic - open to tampering by anyone, not just Comcast, and for many less innocuous reasons than the one for which you've chosen to do so. But with Let's Encrypt, browser manufacturers, and friends leading the charge toward TLS everywhere or as nearly so as is practical, and with most sites that most people use already employing TLS, the attack surface is closing for even an other-than-innocuous variant of your notification methodology. Of course, that also means that that methodology itself is reaching a natural end-of-life, as it cannot work anywhere that TLS exists, and the majority of the web where it does exist continues to grow. If this low-latency notification scheme is of unique value to your business, then now is the time to consider replacing the outdated technology that underpins it with something which will continue to work reliably over the next decade or two.
All that said, I appreciate your decision to engage in this forum. That's unprecedented in my experience from someone in a position like yours, and I wouldn't mind seeing more of it.