Once the soundbox of an archtop guitar or a mandolin has been closed there is very little access to change anything in the construction of this guitar. The tone-bars are not accessible anymore and the shaping of top and back can only be altered from the outside.
John Monteleone reveals some of his secrets how he vastly influences the sound of an archtop guitar or mandolin after the soundbox has been closed. I found it exceptionally interesting to hear which factors besides the thickness of the top and back change the sound in which way. I am sure you can convert some of the knowledge to other types of guitars.
http://www.customguitarvideo.com/2009/10/12/john-monteleone-02/
Enjoy,
Holger