Labs floating IPs can't be reached from labs instances.
For example, my instance util-abogott has a public ip at 126.96.36.199. It also has a firewall policy that should allow ping from anywhere. I can ping 188.8.131.52 from my laptop, and from iron. I can not ping it from within labs, though, not from a labs bastion or from another instance in the same project (testlabs).
This is old news -- we have a few hacks in place with dnsmasq so that some .wmflabs.org lookups resolve to internal IPs in order to route around the embargo on contact between labs instances and floating IPs.
But -- I'm not clear on why this isn't possible. I can't think of security reasons to block such traffic -- maybe it's blocked by accident, or maybe such routing is hard due to the labs network topology. Supporting routing to floating IPs would allow us to rip out some hacks and also promote adoption of the new labs DNS (which lacks the equivalent hacks at the moment.)