Well, after delving into some of Apple's open-source code, I have solved this issue (at least for Snow Leopard). By issuing the following command:
launchctl unload /System/Library/LaunchDaemons/com.apple.kextd.plist
You can update kernel extensions, delete their cache file(s), and reboot; the extension caches will be re-created at boot time. I still need to test this for Leopard, but for Snow Leopard it works just fine.