Ceiling Design Issues

First, let me thank all you fine gentlemen (and ladies??) for all you do for the recording and studio construction community, this resource has been an unbelievable help to countless newbies, myself included. Let me preface this question by saying I don't know whether this belongs here or in the construction forums, heck it could even go in the acoustics forum. A little background, I work with a highly portable recording rig that handles mostly rock bands and they all like to turn up (who doesn't). I have been doing demo sessions for touring bands for about a year and recording live albums for five. I just finished a semi-live recording (you know, no audience) for a band to get a handle on just what they sound like live. I have been using portable gobos I machined and built to get isolation and they serve me well. So, the question is if I were to use a set of gobos making an enclosed room around the performer and used a heavyweight cloth with reflective panels attached as a ceiling at 12' under a 20'-50' actual ceiling what would be going on in terms of low range frequencies because as far as I can tell the sound reflected by the actual ceiling and then reentering would be reduced somewhat, but what effect does that have on the calculation of modes and other properties that one would use treatments for? I know it is an odd question, but I am looking for the best way to neutralize the unwanted acoustical artifacts of a larger room and this is the best solution I have found. Thanks for all you guys do. Jay