A retrievable re-direct (that worked) is one of the great debates in tree climbing. The image below is taken from the Treeworld Forum (circa 2008) andshows the use of a DMM Revolver in a clever re-direct set-up. Note the use of the Kong Minis.
To be truly functional the ideal re-direct should allow the climber to attach it mid line. eg use a swing sided pulley - or a Revolver. DB Tree have anew re-direct planned for release early 2010 which utilizes two Revolvers along the same lines as the Tree World image. One Revolver is spliced permanentlyinto a 1 meter Apsiring sling sheathed in a rope outer for added protection. The other end has a larger splice so the Revolver can be removed and usedfor other purposes when needed. Prototypes are already being tested and pricing is not yet confirmed - it might be just the answer climbers are lookingfor.