Looking up stuff regarding alternators and came across this thread.
I say I have to strongly agree with Big Pistons. You do NOT want to deal with a cabin fire. I sure hope I never have to experience it.
Even if you're just getting a bit of smoke coming out from under the panel you have no idea what the status is. There could be flames which are out of view from you and the fire could be already self-sustaining. The fire might not be self-sustaining at the point you first see the smoke and turning off the electric power could be the difference between the fire going out and the fire continuing to burn - taking 1 second longer could be that difference. Even when you have hindsight you still won't know exactly how it would have gone if you reacted differently. Do you really want to bet your life on being able to make that one transmission? - the one thing that does absolutely nothing to put the fire out and could make the fire worse. If you had the benefit of knowing exactly how the fire would progress and what the consequences would be for every action you take then you'd be able to determine if you do or don't have time to make a radio call instead of dealing with the fire. You don't have that benefit so why are you going to bet your life on it? I don't think people realize how bad a cabin/electrical fire is.
Like was mentioned previously, switching the ELT to on is an excellent idea. When ATC loses you on the radar and then sees your ELT come on I think they will understand that you might have an emergency - I mean, you just switched on your EMERGENCY Locator Transmitter. You should do that AFTER you have dealt with the fire.