Suitable host to ping for WAN connectivity test?

What would be a good host on Comcast's network to ping for a WAN connectivity test?

I have a few customers on Comcast with multiple WAN links for fail-over operation, and the firewalls use ICMP pings to check the status of each WAN link. I've generally used each WAN link's gateway IP address for this, but I recently discovered that in some cases that gateway will respond to pings even if the Internet is actually unreachable. So I think the gateway IP address might be insufficient to detect a downed link.

Other forums, like at Serverfault, have suggested pinging 4.2.2.1 (etc.), but it seems unnecessary to ping all the way out of Comcast's network.

Is there a good, stable host on Comcast's network that can handle round-the-clock pinging, that Comcast wouldn't mind its customers using, that could reliably indicate whether the link was up or down?

Even better, what might Comcast itself suggest I use for this?

Thanks!