It would be cheaper to get a crop body + a less-super tele, assuming you can live with the extra noise, than to try do BIF on a full-frame. Save some weight, save some money, and find new perspectives with the glass you already own.
Besides the 1D X, Joel does have a 1D IV with a 1.3X crop factor. Half way to the 1.6X crop body.
5DS R, 1D X, 7D, Sigma 10-20mm f/4-5.6, 24mm f/1.4L II, 16-35mm f/4L IS, 24-105mm f/4L, 50mm f/1.8, 100mm Macro f/2.8L, 70-200mm f/2.8L IS II, 100-400mm f/4.5-5.6L, 580EX-II flickr