Russia's space-based early-warning system may be in somewhat better shape than I thought. One of the satellites - Cosmos-2379, deployed on the geostationary orbit - appears to be a second-generation satellite, which changes the picture quite a bit.

These satellites, known as 71Kh6, were developed for the US-KMO system, which was supposed to provide almost global coverage of missile launch areas (the first-generation system, US-KS, was designed to monitor only ICBM launch sites on the U.S. territory). The 71Kh6 satellites are believed to have "look-down" capability, which allows them to detect missile launches against the background of Earth.

Cosmos-2379 is deployed above the point 24W on the geostationary orbit, from where it can provide support to the two satellites on HEO and monitor most of the North Atlantic. I have some interesting clues about the shape of the coverage area and I hope I'll be able to post them soon.