I don't know about resistance really but i know with many other games if you are not signed into psn because you didn't update, your system can still connect to the internet, and there is and update available you will be told of so and asked to update the game, only the game not the system, i have done this a couple times with my offline system.
with other games you can get the patches somewhere online (hopefully) and install them like so:
1. Unzip file "Lair.Game.Patch.zip"
2. Copy the entire unzipped folder "00000938" via FTP to your PS3 into "/dev_hdd0/vsh/task/"
3. Power down your PS3 and disconnect your Jailbreak USB Device
4. Reboot PS3 (without the jailbreak)
5. Goto GAME menu
6. Select menuitem "Install Demo (www.XXXXXXXXXXXXX.xx)" and start it
7. When the install is finished, power down your PS3 and reconnect your Jailbreak USB device
8. Reboot PS3 with Jailbreak
9. Play updated game via Backup Manager
dunno how many games this can be done with but i've seen a few updates floating around here or there...