It is easy enough to expose the ReceivePack to Groovy, but I don't quite see its utility.
 Can you give me a code example of what you hope to achieve?

I'd like to get the pushed commit messages but perhaps there's another way of getting
them.

Take a look at:
https://github.com/gitblit/gitblit/blob/master/groovy/sendmail.groovy

I see, so its using JGitUtils.getRevLog and gitblit.getRepository. Using the ReceivePack
seems a little easier:
    for(ReceiveCommand cmd: commands) {
        RevCommit commit = receivePack.getRevWalk().parseCommit(cmd.getNewId());
    }

Nice example. You convinced me.  I'll push the receive pack to Groovy.

v1.2.0 has been deployed.