Yeah I don't believe there is an effective way to do this.
Only thing I could think of to try (and this is pretty hackish) would be to remove all the existing images from the /Library/User Pictures/ directory, and then replace this file:
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/UserIcon.icns
with your own in icns format with the same name. I think (really not sure) that OS X will use that image if it can't locate any in the User Pictures directory. You'd have to do some very careful testing on a few systems that would be OK to break to see if that even works.
Also, no guarantees this would continue to work through point release OS updates, and especially under El Capitan. Those User Pictures will probably get put back in place.
