Why use HAProxy at all?
http://www.igvita.com/2008/12/02/zero-downtime-restarts-with...
It's really a great piece of software. Kudos to Willy.
PS - you're also correct about the nginx->haproxy->apache. nginx makes a fabulous front end and I just plugged in Apache/Passenger where Mongrel used to be. I like that 1) I can easily plug in something else in the future and 2) Passenger on Apache is very stable. Nginx support is newish and I'm running stripped down Apaches that only do Passenger, so I'm not too fussed about it.