Found it.
I cloned the curb source and grepped for :
last_effective_url
In the function below it was the equivalent for the response code, in curb_easy.c, line 2435.
Note to self, "Use the source Luke"!
UPDATE:
The answer is response_code
In my case the code looks like so:
c = Curl::Easy.new(HOST_NAME) do |curl|
curl.follow_location = true
curl.head = true
end
c.perform
puts url + " => " + c.response_code.to_s