Just wondering what types of solutions are available to handle patching of PC/Mac games that don't have any sort of auto updater built into them.

In windows do you just spin off some sort of new install shield for the game that includes the updated files, hope you can read a valid registry key to point to the right directory, and overwrite files?

If so how does that translate over to Mac where the game is normally just distributed as straight up .app file?

Is there a better approach than the above for an already released product? (Assuming direct sells, and not through a marketplace that features auto-updating like Steam.)

Are there any off the shelf auto-updater type libraries that could also be easily integrated with a C/C++ code base even after a game has been shipped to make this a lot simpler, and that are cross platform?

Also how do auto-updaters work with new OS's that want applications and files digitally signed?