Well, basically yes, but I don't think that `sphinx_base.tplx` currently extends `base.tplx` (at least I couldn't find the appropriate statement). 
On the other hand, the inheritance list in `sphinx_howto` gives:
`Inheritance: null>display_priority>latex_base->latex_sphinx_base ` so I guess it was intended to use `base.tplx` like you proposed.
I'll check, and update the `sphinx_base` if possible.