This is my naive take on cutting an oval:
First step is to make a set of templates, one for the soundhole itself, one for each border of the channel. and to include registration holes on the templates to insure they line up. These material for the templates would be the 1/8" or 1/4" aspen/poplar/whatever hobby boards from HD or the like - they're easy to carve. At the end of this step, there would be N templates with oval holes in them (progressively sized). To make these holes, I'd print out the correct sized ovals, and then trace them onto the tempate material using carbon paper. I do a lot of relief carving, and I'm used to working this way.
Second step is to cut the soundhole. The template is registered on the glued up soundboard, using the registration holes, and pinned & clamped. The middle of the hole is easy - it doesn't have to be perfect. Can cut a few holes in the soundboard material with forstner bits, or anything. I would plan to come very close to the edges with chisels/gouges, and then finish up to the line with a round sanding stick, i.e. an appropriately sized dowel with sandpaper glued to it. I've never owned a circle cutter, and a lot of the holes I need to cut for carvings, etc, aren't round, so this is also a familiar thing to do.
The next steps are the trickiest - cutting the channel. The key here is cutting the boundary lines first, using the templates to make sure that I don't cut into wood I want to keep. The cutting itself would be done using a knife with a depth stop, following the template. I'd make the templates such that they came just short of the applicable boundary. (Remember, that the inside and outside boundaries are two different templates, so they're done sequentially.). Once the boundaries are cut, the channel is cut using a 271 router plane, or with a gouge and needle-files (if the channel is too narrow to use the 271 effectively).
Obviously, this wil require some practice to get right! 
I think the method could be adapted to working with an electric router as well, but I like to work with handtools, that's what I'm used to, so that's what I'll try first. I think the key, even with an electric router, is making the tempates. They need to be formed correctly, but you can use any way you're comfortable with to cut oval holes. (That's all the templates are...)
Does this make sense? I'd love to hear comments.
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