No! No! No! No! No!
"...jump the battery with 12 volts and let it charge for a bit..."
I would not leave a 6 volt connected to a 12 volt for any length of time.
I have jumped a 6 volt system with a 12 volt system before without any misfortune. My procedure is to connect the positive to positive and connect the negative to the 12 volt only. Then, when you are ready to fire up the 6 volt starter, have a helper touch (not connect) the negative lead to a good grounding point on the 6 volt.
If you got an old Chevy with a masher starter, go ahead and mash down on the starter before touching the negative lead to the 6 volt groung point. If it is key or button contact for a solenoid, turn the key or mash the button then touch the negative lead to the 6 volt ground. Make sure the helper knows that the jumper lead will generate a few sparks on contact with the ground point. It ain't too much to put up with, but it can be a surprise for anyone who has not seen a few sparks before.
Also, make sure the helper wears a good pair ofleather gloves and is not wearing rings, watch or conductive stuff, as a precaution. A thick pad on the bumper helps reduce the incidence of dancing, too.
Leave the jumper lead in contact ONLY long enough to crank the 6 volt and get it started.
If you are fussin' with a system with an alternator, good luck.
