Yes, exactly - it has to be optional, since not all clouds support it.
If/when that feature settles down in clouds we can think about a new rev
of the API that offers it for all clouds - but that's some way out.

Adrian Cole pointed out the jclouds way of doing this:

http://code.google.com/p/jclouds/wiki/ComputeGuide#Adding_a_post-boot_script

This is a single file injection of a script. If not supported, fallback to SSH will be required so it can do it.