What latency gets added? Presumably on cache miss they already need to look up "whats the origin for www.example.com" and get info about it. This is just a handful of bytes in that record.
If the new for-every-request latency is noticeable above the noise floor at all then they could be really tricksy and make two connection attempts, one immediately with the default guess and one with the looked up value, and as soon as the first connection succeeds abandon the other. In fact if the majority of the connections needing a retry use the same method, so there are two covering almost all cases, you could remove the lookup and just send out two connection attempts, one with each of those two methods. I very much doubt this would be worth the effort, and it would add load elsewhere that would need to be accounted for in judging the value of the optimisation.