Likewise, I'm in the process of setting up Elastic Bamboo on EC2 for Cloud Haskell and would be very interested in seeing how you've dealt with multiple versions of GHC.
It's easy to parameterize builds in Jenkins based on different values of an environment variable, so Johan and I just have different versions of GHC installed side by side, and then set $GHC_VERSION to "7.6 7.4 7.2 7.0 6.12" (or whatever), put /usr/local/$GHC_VERSION/bin at the front of $PATH, and the right thing happens.