Hi Greg, I don't fit into the builder category you mention but I'll give my version of a solution.
If I understand you correctly you want the guitar to sit higher in your lap whilst playing, without an external aid or fixture.
Well my first thought was that anything external is not going to effect the tone. So long as it's not touching the face of the guitar. Anything you do to the body of the guitar will effect the tone, and volume for that matter.
Making the guitar larger so that it sits higher, and changing the shape, so that it sits at the desired position/angle, is going to change the response of the guitar.
It may depend on how well the guitar is built, as to the degree of difference noticed.
How much difference and in what areas of the tone spectrum, I think is in the hands of the builder. It would depend, I suspect, on materials used, bracing design, increased volume in the body.
I'm talking full size guitars. If you are talking box style guitars, I'll wait for the photos.
Why can't you just put a strap on it, adjusted to the desired height. If you watch the Gitty Gang Show, that is what they do, all the time. A piece of cord would work, nothing fancy.
Always interesting to see new ideas.
I have played around refurbishing guitars for about 20 years and from what I have seen online and at guitar shows I doubt it would make much difference to tone. I base this on having seen harp guitars up close and online and there is not much tonal difference between the ‘normal’ shape and having an extra arm to the instrument. Also I own travel classical guitars with tiny bodies and normal sized classical guitars - the volume is less but the tone is similar.
This is an example of YouTube showing how a classical harp guitar sounds with the extra volume of an additional arm which is continuous with the body cavity.
Good luck with your project and remember to post photos :0)
If I need an excuse for anything it's almost always; "curiousity".
I have been very interested in this concept as well. I keep visualizing what one might look like and although I am always a "form follows function" kind of guy, I can't help but think it's gonna be butt-ugly if I do it. Everything I make is butt-ugly.
Looking forward to progress that you can make with it.
Hi Again Greg, just to let you know that you should have no issues with bracing a custom classical guitar. There are many bracing variations by many Spanish luthiers that you can utilise that are based around the fan braced model. It would be a matter of adding a brace to the fan design and lengthening the transverse braces, if building a larger guitar.
Jose Feliciano had large classical guitars built for him, based on the size of a dreadnought steel string. This was for two reasons. One, to make it more comfortable to play, and two, to get deeper bass tones. His guitars were 16" across and 4.75" deep.
The photo of the deep guitar above, I'm told by the owner, was built for Feliciano. The other standard classical guitar is 4" deep.