Some tests show, that exporting mesh animation (Armature only) as Alembic .abc can be a workaround. Because when it is imported via Mesh Sequence Cache modifier, Blender starts thinking that my mesh is always represents its basic form, even though it is visually transforming as by usual Armature.
- Disable rendering of modifiers (camera and eye pictograms), that you do not want to contribute deformation to the exported object. Basically, .abc cache will replace your existing deformation by Armature
- File->Export->Alembic (.abc) and then choose necessary options on the left
- Add Mesh Sequence Cache modifier and load exported cache, then choose object path at the bottom of modifier menu
- Put up Mesh Sequence Cache modifier in the stack, so it would take place right after all unwanted deformation. Disable all modifiers before Mesh Sequence Cache and enable those, which are after (see pic)
Work is done, now Blender define deformation only made by Displace.
If you want to edit your basic animation, you can enable Armature, disable Mesh Sequence Cache and simply re-export new cache. That is why I don't recommend to delete modifiers before Mesh Sequence Cache