OK, as to installing the (currently) final security update for Mountain Lion, I figured out how to do it IF you have access to a newer system which supports Mountain Lion directly. I know it doesn't help if you have no access to such as system, but most people surely can find a friend with one I would hope:
0) Make sure you have a backup. Make sure you have a clone, clone of your own. Really. Otherwise you might panic, and you don't want to do that.
0.5) Install your Joo Janta 200 Super-Chromatic Peril Sensitive Sunglasses.
1) Make sure you have your customized ML installer ready and working for the destination machine model in question.
2) Using an external drive hooked to a machine natively running ML, boot into the recovery partition (or otherwise use the method of your choice) and install ML.
3) Boot the newer Apple-Supported machine into ML on the external drive and, if you wish, use migration assistant to migrate your data etc. etc.
4) Fully update that installation of ML, using the Apple-supported model the entire time.
5) Once you are satisfied everything in fully updated (including the latest security update) looks good, hook that drive up to your older unsupported system and boot from your hacked ML installer.
6) Install the files as instructed by ML/MacPostFactor onto the external drive. If everything goes properly you will have a fully-functional and fully-updated system at this point, but a plist tells the system you are on 10.8.4. This is incorrect and you can find instructions back in this thread somewhere for correcting that if you wish. It's a simple plist replacement IIRC.
7) Once you are satisfied everything is correct, you can then clone (using CCC, or whatever you prefer) that external drive back to the internal drive on the destination machine. (Remember, you followed step 0, in case something goes wrong.)
This has worked reliably for me across a few models. It was important, in our case, that we get that security update in place since it's a networked school environment and I'm twitchy about not having the latest security updates...
Hope that helps a few of you.