A few things. . .
I'm irritated that Britax customer service brought up tether anchor limits in cars when I called earlier today to try to get a replacement for my boulevard since this is in the Latch manual. What is the limit for tethering to a third row seatbelt as per the latch manual? Is there a reasonable assumption?
Also, I have mine tethered like this right now- to the male portion of the buckle. I was wondering if I need to do anything to make sure the buckle doesn't slide on it's seatbelt webbing. It's just a lap belt. I don't know if I'm being clear.
I think Britax is trying to cover their behinds by reminding folks of their VEHICLE tether anchor limits. This is still a gray area that techs can only educate parents on; there's no clearcut answer. Everything I know and believe says it's better to continue using a top tether anchor even after the limits have been reached, because the tether anchor is not what's holding in the carseat, the seatbelt is; the tether anchor is just a supplement, and it will do the job it's designed to do even if it fails at the height of the crash forces.
There is no "limit" mentioned when tethering to a vehicle seatbelt. My thinking is that a seatbelt is designed to restrain THOUSANDS of pounds of force, to hold back an adult. A seatbelt as a tether should have no problem supplementing the seatbelt already holding in the carseat.
Using a lapbelt is the easiest belt-tethering method, IME. Lapbelts will automatically lock (except for the rare kind that need belt shortening clips). The kind with a sliding latchplate (most common type) will lock when the sliding latchplate is held flat against the belt. That's exactly how it will be when a seat is tethered to it.
Here's my two pictures, one showing tethering to the buckle tongue (obviously unbuckled) and one tethering to the buckled webbing: