I've had a crack at this. Satnavs that use a mini usb socket run on 5 volts. So i tried connecting the sat nav straight to a 6 volt battery. It worked! the 6 volt battery showed 6.5 volts on my meter. i cut off the plug from an old car charger to make the lead.
Now when fitted to the bike and the dynomo is in full swing the battery could be reading 7 volts and maybe slightly more so there is the chance that this could be too much for the sat nav, its at this pointo where I reach the end of my electrical skills. maybe someone could recomend something that will hold the voltage to 6 volts.
You could of course just upgrade to 12 volts but i guess that is not what you want to do.
btw. as I was doing this I got the polarity reversed by mistake! the satnav didnt work but when connected the right way round it came back to life.