You can use the Iray emitter shader on the dome. The quality isn't as good as using an HDRI, or Sun/sky, but sometimes it is what you need for your image.

Make the color pure white and add the texture map from the diffuse color of the dome. Make the temperature 6500K. Add a few zeros to the lumminance (Probably 5).

That should give you enough light to render. It is also how this image was done.