I am up for just filling the underways with nests of intelligent giant spiders. It doesn't matter where you burrow in if there's giant spiders there to notice you and report your presence through spider radio.
Actually, hey, spider radio. The end points are We with that Amber spell on them so they can relay words, and spread out We-nests can be used to carry the signal. When they reach the end of one We's range, they can just pass it on to the next one. The Dwarves could pay them to keep a few bodies in the relay at a time with goats or something, and in exchange they'd have wireless communication infrastructure.
You are now imagining Belegar holding a Chihuahua sized spider to his ear like a cellphone.