Getting Chromecast to work across subnets
As far as I can tell the issue the only issue preventing Chromecasts from being used from other subnets is one of discovery which is dealt with by multicast UPNP packets which unfortunately have a TTL of 1. Instead of getting my router doing all of the usual multicast shenanigans and adjusting the TTL as you suggest, I wrote a python script to advertise my Chromecast on the other subnet. It is available on github.