Yes, for the brief period that your STB is connected, your regular Internet service would be down. I was thinking that the STB would grab your external IP address and start downloading the guide pretty much immediately (if it's going to work at all.) Essentially, it would be a device on the Internet without a firewall in front of it. Not something you'd want to leave on there long, but it should only take five minutes or so.
However, you made me think about it again, and that may or may not have issues. Verizon will only lease an IP address to a device with a different make address when the current lease expires. That could be up to three hours. Still, your STB isn't usually behind a firewall anyway, right? There's a splitter that runs the coax directly into the STB and into the router. Only your PCs are behind that. So, Verizon must give an IP address of sorts to the STB.
So maybe use a splitter to keep your router connected, but do the split as close to the ONT as you can. I'd still vote to bypass that ground block for the test, too. I'm just wondering with a short run and the house wiring not in the picture if you would have better results.
