I ended up giving avahi-daemon a bash (just for 'fun'). The Chromecasts are in 192.168.1.1/24 and they appear sporadically in 192.168.3.1/24. But, more-often-than-not, the appear and then disappear, and then I can't cast to them reliably from 192.168.3.1/24. If anybody has any experience of battle with avahi-daemon, then I'd appreciate the guidance. I've managed to get it working in ddwrt before, but so far, haven't succeeded here.