Hey, builder fellows, I LOVE what you're doing here. I will be lusting after one of these axes for studio work for sure.
I have an amplification suggestion you might not have tried... I use an older version of this AT mic on my reso live, and it's very good, open, airy, and gets the reso sound, and with minor adjustments in positioning can be "tuned" for relative bass/ treble response.
http://www.audio-technica.com/cms/wired ... 05bdc46d6/
The are aimed at the saxophone and acoustic instrument market, but I bet the capsule could be permanently mounted under the cover plate or even under the cone inside the body and give acceptable, feedback-resistant results.
The one I have uses phantom power, but other than that is fairly trouble free and presents a good acoustic reso tone to the soundboard.