Modular Room-w/in-a Room HT? Think's a good idea?

Agreed. That's why recording studios are built that way; with sonics the priority and keeping out external sound. Those rehearsal rooms are not concerned with keeping the true sonics of the instrument, the concern is a quiet room inside and out.
I think, if you've got the budget, it could become a good music room. The alternative is constructing secondary drywall with sound absorption and wiring which amounts to building an entire room, which may cost more.

That's why a consultation with a contractor who has built studios may be a good way to start.

Lowboy, good sound inside has got to be a relative term at most, up to some level of interpretation. I think, for all intents and purposes , the home AV/HT market has pretty much got down what makes for a good sounding overall space in any given relative room dimension and design, and how to treat it at this point. Simply going to any PMI LTD, Lucas THX, Rives Audio website, and similar, etc, can get anyone understanding what amount of diffusion, absorption, reflection, re verb, bass absorption vs relative size, etc and so forth is needed to build into any space for MORE THAN adequate sonics inside, say, hidden panels and corner/bass traps??! That part would be easy.
As for what makes for "keeping the true sonics of an instrument"(?) Lowrider57, I'm not completely sure what that means to you..or to anyone else for that matter. Perhaps, coming from some pro-audio/studio background, you have your own tastes in room acoustics? Couldn't help you there. Ask any number of theater builders vs Audiophiles vs audio engineers, vs home theater enthusiests vs pro magazine reviewers, and so forth, I'm sure you'll get varying options and tastes. And, yet, simply realizing that smaller spaces have less bass absorption vs mid/hi freq energy - which requires more diffusion around room plus treating first order reflections, and corner bass traps, is a must. Medium sized spaces need similar but more absorption mixed in on sides and ceiling. Larger spaces absorb much more bass and therefore need more midrange/hi freq absorption to balance out, plus some diffusion all around. ..basic.
We could go on for days here though.
My point is that that could be worked out formula (with some customer modification(??) upon assembly if desired, again hidden inside fabric pannels internally in any structure, maybe bass traps in corners, and basic. Still MUCH MUCH better sounding than any drywall painted echo chamber or untreated standard domestic living space...I assure you

IMO, the market would be even smaller than targeting audiophiles. I think for many of us, the journey is as important as the end goal. Now my wife, on the other hand, might prefer I spend the money and be finished.

Then again, if the only space I have to play with is in the attic of the detached garage, I might be very interested in installing a sound room complete with base traps and customized wiring.

Ultimately, it all comes back to the price.

I believe targeting audiophiles is probably not the market, actually. I think audiophiles are happy with their $5K pre-amp, speakers and amp's of choice at the present, and some records and spinner plus tweaks, mostly. I don't even think a dedicated construct-able sound-room even would be in their equation, honestly.
However, anyone looking for a full on custom home theater/entertainment space in their home might strongly consider such if it was marketed to them effectively, say, through their local custom installer/AV retailer. If a customer was showing interest in a dedicated space setup, then the custom retailer would stear them to one of their rooms with said "Modular Room system", and show them the benefits, and why said customers NEEDS such a product! And, as you said, Pgawan2b, if the pricing was reasonable and right, it might then work...especially for basement homes back East, and such like. I think it could work, really..and I think more need could be met here than one might think actually. Maybe maybe...

Anyway, actually, this would still likely have to be a fairly involved system to construct. You'd either need the floor to be floating over the existing sub floor, and the sidewalls to rest on top of that, or the whole modular room to be floating on isolation pads/system of some sort.
Also, actually, the construction of floor and wall panels could even be similar in the way they attach to each other? But the ceiling system would prob need to assemble a bit different , I presume? And then also, if you stored all of this in your garage, or storage, or another spare room, dis-assembled, or whatever, that would still constitute some rather bulky furniture to have to deal with, whenever it wasn't in use/assembled.

Still, if you're going to go through the hassle and expense of buying an involved and pricey home entertainment system in a dedicated room/space, doesnt this makes more sense for most, IMO, rather than expensive room construction on existing - furred out walls, resilient channels, green glue, better insulation, permanently re-routing HVAC and lighting, blackening out the room, and still having sound leak through window and standard door openings, and then not truly decoupling the room from the rest of the home anyway? So cranking up your system becomes somewhat limited, due to noise leaking/transfer from all directions. and then of course this idea can be relocated and assembled at another location in future.
Well sounds interesting to me anyway. lol...