I get that's problematic for your modernization efforts, but in that case: eliminate modem rental fees. Bake the fees in to the standard cost of the service and don't let customers use their own equipment. I understand that non-cable competitors don't have this cost to shuffle around, and that this will mean you are forced to either A) raise prices publicly or B) have lower margins. That's your problem because of your technology legacy; don't pass the misery on to the customer.
While you're at it, offer two hardware choices: one with, and one without routing/wireless. I refuse to run a wifi network in my household for your other customers and expect complete control over my LAN configuration.
On the topic of injection: I get that you don't think it's immoral, but hey, 1) most people who understand it think it is totally unacceptable. And 2) the window for this approach is rapidly closing for you as the web moves to SSL everywhere. Give up on this approach now and save face.