Charge/run lights with outboard -- check amps, too
It wouldn't hurt to check the amp output from your outboard's alternator before proceeding. My 5hp OB's alternator only put out 2 amps at crusing rpm, which wasn't enough to do anything with.
Before rearranging wiring to run your nav lights from the outboard, if you're doing a lot of night sailing, what is the total amp draw from your nav lights? (The aggregate draw from my bow light and stern light is only 2-3 amps. You can get a lot lower than that with LED nav lights.) Realistically, a decent group 24 deep cycle battery should power the lights overnight, and you could recharge once returning to the dock. Seems simpler to me than splicing in a switch to chose between battery and alternator power.
One Step Closer
Lake St. Clair, MI