It should hardly be necessary to stress that focusing the sun with a 1000 mm lens through the view finder is really dangerous and one can impart permanent damage to the eyes. Using Liveview in this case is the obvious option as it is better to fry the image sensor than one's eyes. If the image is taken when the sun enters cloud banks and/or is about to descend below horizon, the light intensity is reduced, but one should still be very careful.
The best approach is of course putting a solar filter over the lens for focusing, lock the focus, then remove the filter and snap the picture. A solar filter is dense enough to block most of the sun's rays and lower intensity to a non-dangerous level for our eyes. There are two classes of such filters, one designated for visual observations, which is on the dense side for photography, and another less dense suitable for photography as shutter speeds will be much faster. However, in the last case, reduce focusing to the shortest time possible or preferably use Liveview.
Look for the pictures of the Mercury transit during the NG Scottish meet-up earlier this year - we used solar filters on all lenses.
