Since you can't get the end cap off anyways, here is what you could do (assuming that aesthetics are not too important to you).
Take a wire brush and clean the paint off a small area of the seam between the end cap and the body of the muffler. Then ride your bike to your friendly local muffler shop and ask if they would kindly put a small tack weld on there to stop the rattle. Then use a file or grinder to smooth out any sharp edges and hit it with shot of flat black spray paint. This should be a semi-permanent fix that could always be gound off in the future if needed.