Zeke,
I think we might be at cross purposes somehow here,
(a) While I have no problem with your stated lemma that a nacelle VG would not be intended for actively effecting a stall/spin recovery during any particular event, such devices may be very relevant to determining the aft cg limit for which satisfactory stall and recovery characteristics can be demonstrated. It follows that an MEL-sanctioned absence of such a device would, necessarily, involve consideration of appropriate cg restrictions to the aft limit to retain handling qualities not inferior to the normal installation. Two matters are of direct relevance -
(i) The aft cg limit is determined by a number of factors, including stall characteristics. As the cg moves aft, recovery progressively becomes more critical and eventually may be ineffective, as has been demonstrated in a number of fatal accidents over the years. Such a consideration is, for example, one factor in the typical light aircraft dual normal/utility category certifications having different cg envelopes for each of the two categories.
(ii) For the same reason that an aerobatic empennage design geometry minimises tailplane wake blanketing of the fin and rudder, I suggest (without yet having had an opportunity to look at the devices referred to) that the structure observed by Spinash (presumably an Impulse man/woman - and no doubt quite relieved by the ACCC's decision to permit the QF deal) are VGs - intended to delay, or otherwise control, flow separation - which would blanket, at the very least, the lower fin and rudder region.
Hence, one would expect the sort of MEL restriction to aft CG which Spinash mentions. And a 7-8 pc restriction is not a trifling amount.
(b) I must confess to absolute and total confusion by your second comment. Apart from stating a lemma - that the nacelle is in the wing downwash flow field - you appear only to be restating my earlier observation.
On the other hand ... if I have demonstrated gross technical ignorance in these matters, I would appreciate further enlightenment....