一大群废话我也是服了,只有下面这段话还可以

If you wanted to have a T pose rest pose, and use an armature to pose your pre-posed mesh into a T pose, then it seems to me you could copy your armature, match it to the mesh in edit mode, rig, repose the mesh, apply the modifier, and then delete the armature and swap in the armature copy. That involves one step of posing to match the mesh (in edit mode) then ordinary rigging, the one step of posing into a T pose (in pose mode), and then the extra step of swapping in the copy armature.
It's not the most elegant solution in the world (I'm not sure I'm fully understanding the problem), but it's not that complicated and no constraints need to be messed with.