Can I use a computer memory unit with a 9 Volt battery to maitain codes when I replace battery?

That will work as long as the ignition switch doesn't have to be on for the lighter to work. If it does, you'll be asking that little battery to power all the things that are switched on by the ignition switch. That includes the radio which in itself will draw the battery's voltage down far enough to kill the memory circuits in various computers. I don't mean "kill" as in damage. I mean erase the stored codes and fuel trim data.

I use a small battery charger hooked right to ground and the positive battery cable, but you have to be careful the clamps don't pop off and the positive one doesn't hit anything metal on the body.

Happy to hear you understand the need to maintain voltage in the system while replacing the battery.