Finally got the framing done this weekend. I decided to go with Bluwood studs instead of steel studs, thinking they would give me a better chance at a wall that was rigid enough to not require RSIC DC04 brackets to secure them to the joists above - but no chance, they are very wobbly unsupported. In hindsight it is obvious now that a 26' 2x4 top plate simply deflects to much. I think in order to acheive a true room within a room isolated framing you would want 2x6 top and bottom plates at a minimum, and you would also want to plan for columns as structural elements (ie for lateral bracing)
Here are some pics of the framing - the room looks like a bad acid trip right now with the pink spray foam and the blue stud walls
Because I have a very long room to work with, I wasnt too concerned with keeping the stud walls tight to the front and back of the room. On the left hand side (the interior wall with no spray foam) I was able to keep around half an inch from the cinder block wall. This picture shows my framing inside the door opening - I am planning double "airlock" doors here for maximum soundproofing. When finishing the doorway care must be taken not to short-circuit the two walls here with the door casing. I believe this gap should simply be caulked?
On the right side of the room I decided to turn the studs parallel to the wall. This saved a precious 2" of room width, and also kept the bench walls (from the excavated portion) balanced with the other side. I am going to attach metal corner beams to every other stud to beef up the rigidity of the wall, and with an inch of drywall plus the wood panelling the wall should be pretty solid.
You can also see where I had to cut back some of the spray foam so that the studs were not touching it. In general the spray foam application was very consistent but at the bottom of this wall the waterproofing membrane was coming away from the foundation wall and it resulted in a bump in the foam. No problem with the multitool - just incredibly messy!