After a tonne of googling I wasn't able to find an easy way to change Passenger's spawn mode setting in the default Ruby 1.9.3 Elastic Beanstalk (EB) image.
So I wrote this: https://gist.github.com/timclipsham/5917769
Since EB is using the standalone version of passenger, the script simply manipulates the
/etc/init.d/passenger file (which starts/stops the passenger process) and appends
--spawn-mode conservative to the
Read more about how to customise your AWS EC2 instance created by EB if you're curious.