I am not familiar with the layout of the car, but the idea of modifying the blank panel sounds like an awesome idea. It looks like the only option that would keep the dash looking clean by not having a wire hanging around when the AUX input isnt in use. I would go for that idea.

The thing that I would think ahead about first is if that panel would hold up to constant plugging in and pulling of the plug. If it looks like it can take that type of abuse, go for it. I am sure there are ways to make that modification look like it was part of the car.

However, if the panel is on the dash, you will have a wire there whenever you are using the AUX input. Maybe you can find an area that is more out of the way. I've seen people run the wire into the glove box also.