How: Just stand up a nginx with a single proxy_pass rule at OVH (“the premier bargain basement cloud provider”) or something.
You CANNOT put long polling behind CloudFlare. They terminate long-running requests as, incidentally, a anti-DoS measure.
Because DDoS protection is a priority for you, set up this server at a DIFFERENT provider so attacks on it do not actually take down the main site; you just get degraded live update functionality.