You can use a winch extension as a tree trunk protector. We do it all the time in comps - evil stages where they make you use all your gear.
The only stipulation , in comps, is that the tree trunk protector must be used in such a way to achieve the same, or more, area of coverage on the tree.
In saying that, for the cost of a Tree trunk protector, just get both. It will make your recovery kit more versitile anyway.
Welchy