I was thinking more specifically of the 500mm, rather than attaching the camera directly to a telescope.

I imagine it would work well with the moon (as mentioned).

As far as cheap (or, cheapER) conventional solutions, if you put a 1.4x or 2x converter on it and you can bring that up to 1000mm. Further, I don't think there's anything stopping you from using 2 convertors on the 500mm.

The problem then becomes light sensitivity and shutter speeds required, I suppose.

It does pique the imagination, does it not?