You can buy a device quite cheaply that plugs into the OBD port and has two leads with crocodile clips that you connect to a spare battery / golf battery / 12v household alarm battery, which retains all settings until the new battery is fitted. It has led lights that let you know all is well to disconnect the battery.
There are others that plug into the cig lighter socket but I prefer the OBD type.
Cant vouch for other Cars but on the Omega the positive on the ODB Port is direct to the Battery Positive, un Fused, so my advice if you are going to to connect any form of other Volts to it, make sure its got some form of overload protection or better still fit your own fuse inline to what ever your pluging into it.
As to charging a battery on the car, I have two. Both state that the negative lead should be connected to a strong earthing point not the neg trerminal of the battery, and as far away from battery as possible, although this is dictated by the length of the leads.
Some modern cars have specific positive and negative terminals away from the battery specifically for charging / jump starting purposes.
They say this so as to reduce the risk of sparks when connecting or discounting the Charger Clips to the Battery terminals (Possible explosion if the Battery is venting hydrogen), personally I would always connect directly connect to the battery terminals and take the risk, rather than have potential stray voltage going through the body of my Car.
I always diss one of the battery clamps so as to isolate it before connecting a Charger if charging the Battery in situ (normal the Poss, although Neg will do the job just as well) or as said before if you really wont to play it safe diss both Terminals or take the Battery out of the Car.