If there are any feedback issues, they're usually remedied using the Master Gain and notch filters on the TWA. You're also able to run the TWA in DI mode in which you can select "TWA and DI" or "TWA or Amp". The latter will allow you to use the TWA as an effects unit and will send the signal out to the PA but not through the soundhole if you don't require hearing the guitar through the sound hole onstage.
Regarding the feedback issue/question:
In a situation requiring there need for monitors I can't see the point in trying to make use of the acoustic side of the TWA. This would be where I would use the TWA as an effects unit as described on Page 14 of the PDF Users Manual (should I wish to make use of it on stage which I don't because I have other equipment for just this). The use of the sound hole plug is probably a good idea as the TWA (due to its properties as a sound hole projecting unit) will probably increase the risk of feedback.
As good as the TWA is, it is still bound by the laws which apply to feedback etc. Sorry to disappoint you if you thought you were buying something with a magic wand properties i.e. one wave makes feedback problems go away.
As a purely acoustic effect, I wouldn't want to be without it. On Stage I wouldn't want it on my guitar due to the increased risk of feedback. It may be better to then use it as you'd use any other effects unit. Connected by a longer cable, and not "sticking" to the guitar.
I hope that this has been of help.
What a Wonderful Gadget. You can take with you, all the special sounds of a sophisticated amplifier, without the burdensome boxy weight and space of what that entails. Excellent! Beautiful! Congratulations on a well designed, ahead of its time, musical sounds enhancement devise.