Short of buying a better body, the way around this is apparently to fit a special chip to the lens mount, which tells the camera that it's set to an arbitrary aperture and manual focus. Some info about these chips here:
These seem to work out around £20 already fitted to a T2 mount, but I'm a bit skeptical of their reliability, they look like the chip would break off if you breathe on it hard. And to be honest my 500mm lens isn't a good one, it may not justify the expense, especially if it's likely to cause problems.
Does anyone have any experience of these mounts?