The first step is to create an Apple Disk Image File (DMG) distribution in a GUI tool, I used DMG Packager.
Then create a directory that will contain all the files that you want included in the DMG file. The best way to do this is to mount the DMG file you created and copy the files into this directory. Then move the .app file created by BitRock into this directory.
Then run the hdiutil utility to create a DMG file
$hdiutil create /tmp/tmp.dmg -ov -volname "RecitalInstall" -fs HFS+ -srcfolder "/tmp/macosxdist/"
Finally call hdutil to convert the writable, to a compressed (and such not writable) DMG
$hdiutil convert /tmp/tmp.dmg -format UDZO -o RecitalInstall.dmg
So now each time before you build a new distribution with the above commands, just move the new .app file into the directory containing the files to be added to the DMG file.
I've already added this to the makefile so after BitRock creates the .app file the hdiutil command is called to automatically generate the new DMG file.