With some experience you can get the assembled back frequency into the right ballpark using 'free' plate tuning methods, but it's far from exact. Any change from what you've done in the past makes it harder to predict, so a new shape or bracing system is a new learning experience. In the end, if you're after some exact pitch, you'll probably need to reach inside and shave braces or some such.
Some years back I tracked Dana Bourgeois through his tap tuning process on a guitar at a GAL convention. I tested the modes of the assembled box before and after his tuning processto see what he had done. Basically, he started with a top that was a bit too heavy and high pitched, and shaved the edges to lower it's modes to get them to couple more strongly with those of the back and the air. All he used for 'instrumentation' was an educated finger, and his ears. The 'ears' part is easy to understand, the 'finger' thing a little less so.
His method is to tap the top in the bridge area, either in the center or where the end of the bridge would be (this is _before_ the bridge is put on). He also tapped in the lower bout on the center line between the bridge location and the lower edge. In each case he would tap and allow his finger to rest lightly on the top. What he was feeling for was a 'kick'. I'd always found his written descriptions of this confusing, but after seeing him do it, it became easier to understand.
Suppose you have the 'main back' resonance tuned to be something like a semitone higher in pitch than the top. When you tap the top it starts a compression wave going toward the back. When that wave hits the back deflects pretty strongly, since it's close to the top resonant pitch, and and returns a fairly strong wave toward the top. When that return wave hits the top you feel it 'kick'. Tapping in the lower bout between the bridge and the lower block checks the coupling of the top 'long dipole' mode with the 'A-1' air resonance. On Dreads it's possible for the top 'cross dipole' to be close enough in pitch to the crosswise air resonance in the lower bout, which may be the 'A-2', although I'm not absolutely sure of the terminology there. If nothing else, more coupling between the low-order modes tends to give you a more 'interesting' sound, ans probably increases efficiency as well.
The important thing is that Dana has figured out a reasonably reliable way to get good coupling going on his guitars without the use of any technology, and without even knowing about the resonant modes that are involved. His methods are not foolproof, in part because he doesn't really know what he's doing. For example, if the top 'cross dipole' mode is to low to begin with, no amount of thinning the edge will get it to couple with the 'cross' air mode. OTOH, thinning the sides in the area below the waist might help in that case, by increasing their compliance and lowering the air mode pitch. Dana relies on getting wood with high cross grain stiffness to make this work, but, again, he doesn't actually measure this, except by feel. This is one of those cases where 'tradition' could be helped by a little 'technology'.
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