The basic logic is simple but evil. I'm simply mapping BGP next-hop to a set of weighted servers. A server is then randomly chosen from this pool.
I'm not doing this for -all- prefixes and POPs - it is only being used for two specific POPs where there is a lot of peering and almost no transit. There are a few issues regarding split horizon BGP/DNS and request routing which I'd like to fully sort out before I enable it for everything. I don't want a quirk to temporarily redirect -all- requests to -one- server cluster!
In any case, the test is working well. I'm serving ~10mbit to WAIX (Western Australia) and ~ 30mbit to TORIX (Toronto, Canada.)
All of the DNS based redirection caveats apply - most certainly that not all client requests to the caches will also be over peering. I'll have to craft some method(s) of tracking this.