Guy, I have been following this threradhere and at TB, someone over there posted a pic of a helilcal screw, and without wanting to derail at all was wondering if anyone has any experience of using screw piles in stabilising trees in any way not just in the specific example here. In oz there are companies using screw piles for exactly such an application.
Screwpile Australia - Applications Tension Anchors
I'll add my thoughts to the questions in your original post, but need to know whether you were expecting/or feeling that failure was most likely in the stem or in some of the larger limbs? If it were the limbs that had your focus then Cobra or rather TreeSave would be my choice a dynamic support sysytem or fall arrest for the major limbs then massive efforts to improve and invigorate the soil and roots.
Maybe some diagrams might help clarify things a bit let me try with the pic you put up.
