To my mind, and I'm not being a rules whore in this case, but if you have an NA6 and install an NA8 subframe (they are interchangeable), this would not be a non-compliant configuration.
Unlike a NB subframe in an NA, which requires the new rack and provides better suspension geometry, the improvements would probably be negligible.
Still, I'd be curious to know whether there's an improvement in lateral stability of the rear pick-up point of the LCA with that bracket between those mount points.
Would it be grounds for a potential protest? Perhaps, but then I"d say you'd have a case for an error/omission similar to the rear subframe bits (like what Dave W. references is missing in the GCR).
At this point in the rules progression, I'd say write a letter to the CRB asking for a rules clarification about both the front and rear subframes.