I just found this way to do it, with some SERIOUS help from @kcase who tried in vain to educate me about sed :-)
mdfind -onlyin . kind:Omnigraffle Leo -0 | xargs -0 zgrep -r “Leo” *.plist
mdFind is the Spotlight find. This finds all omnigraffle documents in your current directory that contain the string “Leo”, then pipes those filenames to zgrep. Zgrep recursively drills down into your omnigraffle directory, finds the zipped .plist file, unzips it and prints the string with “Leo” to the console.