From what I've seen, load releasing hitches such as the Radium are to release loads, not lower them, well, not lower them far at least.
There are a couple of really interesting ways to set up rope so a climber could be lowered from the ground, conditions in the tree permitting. One way is to cinch the trunk of the tree with a short piece of rope. One end of the short rope would have a F8 with a bight (a loop knot) in it and a biner clipped in to the bight. Wrap the rope around the tree and clip the biner on to the rope to form a cinch. Tie another F8 with a bight just above the biner and clip either a Grigri, even better an I'D, or a micro-rack to the second F8 loop with a biner. If you use a micro-rack you have to tie it off, but it shouldn't 'bump' when you untie it.
The climbing rope goes over the TIP, back down to the ground and through the grigri, I'D, or rack. This method allows a climber to be lowered all the way to the ground, from the ground, IF the climbing rope is long enough. E.g. If the climb goes 50' high, you'd need at least 150' of rope. 100' to go up and back down the tree, and another 50' in reserve to use for lowering.
I've even tied two ropes together (just above the friction/locking device) to form a 'longer' rope from two shorter ropes. Of course, if the climber has gone over/around limbs, it is possible that he could come down on the wrong side of a limb or get hung.