Building an oscillator and feeding it to a charge pump or step up transformer is easy. Building an insulated electrode that can withstand the heat, pressure, and shock of igniting the charge is much trickier.

First off with the electric ignition thing, are we talking metallic cartridges or muzzleloader (or non-cartridge breech loading like the Harper's Ferry rifle)?

Combustible cartridge.  Thin cardboard, like a firecracker, but it's doped with nitrates or something, so it burns up like the old flash paper.  I suppose it would work with plain cardboard, but having it burn up would be nice.  The bottom is crimped like a shotgun shell, and the top rolled down onto the bullet, with a bit of wax for waterproofing.  Just like the picture, except I don't know a good way to show the crimp, and I'm not really good at drawing at this point anyway.

To fire, the hammer hits a piezoelectric crystal, which sends a jolt down the firing pin at the same time it's punching through the wax at the center of the crimp far enough for the spark to get to the powder.  Powder goes off, out goes the bullet.  Ideally you use the flash paper, so that you just open your cylinder and stuff in a few new rounds.  The ignitor might be gotten from an electric cigarette lighter.  Those use just this system, you depress a spring which pops up and hits a piezoelectric crystal, making enough electricity to create a spark.  Black powder might need a bigger jolt, but ganging a few together might work. 

These cartridges will probably not be strong enough for a semi, but will be made fairly easily.  You might even later make combustible plastic cartridges that will be strong enough to cycle in a semi, instead of a punching in, the firing pin hits a spot in the back that's been doped with another chemical to make it a "primer" that will go off with the spark.

Eh? The basics of the paper cartridge revolver system have been working in cap and ball revolvers for some time now.

Personally, I'd just skip the whole thing with having the electrode pierce the cartridges at the moment of firing and just have a dedicated spark plug on each chamber. Maybe have the piezo crystal mounted directly on the hammer and strike directly against the spark plugs.

I've got doubts though about how hot a spark you can get from a reasonably sized crystal. They have trouble sometimes igniting butane, how well are they going to do with black powder?

The paper cartridges were made of a material called flash paper that is well known to burn completely.  It was soaked in a nitrate solution or something.  Some artillery uses a plastic material for cases that burns clean enough, it's used in the M1 Abrams' main gun for one.

I think that would just be cool to make your own primers, scary, but if you could do it right... I dont know how you would reuse primers, I think once they were hit with the firing pin and had the dent... Dont know how that would work.