I am more than a bit uncomfortable about the stability of suspending the RSIC-1 clips and the furring and 2 layers of gyprock from what is essentially a Tee Bar grid system similiar in design to the ones that hold up acoustic tiles.
I wasn't suggesting you use a grid system designed for acoustic tiles. Of course that's unlikely to work. The main idea being offered was to hang the ceiling
below the steel beams rather than between them. The reference to the pictures in the PDF were to illustrate the idea and see the components involved in building a suspended ceiling.
If you have a close look through the various Rondo PDF's -
http://www.rondo.com.au/rondo/rdo_liter ... 1&status=1 - you'll see there's all sorts of systems described including ceilings you can walk on. A supplier like Rondo, just offers all the components - it's up to you to select the parts most suitable for your application.
Suggest you call the supplier of the RSIC-1 mounts, explain you want to suspend them from the joists at varying heights and ask if they have any suitable
adjustable hangers or know anyone who does. This is a common situation - you won't be the first person who wanted to suspend RSIC-1 clips - the parts must exist in the U.S. or Canada.
You could use timber, but it's fairly cumbersome to work with in this situation - you need some adjustment to get the furring channels in the same plane - whether it's sloping or flat.
For my own suspended ceiling, I used steel brackets from the hardware store. They're actually framing brackets designed for joining timber underneath decks and so forth. They have a slot as well as holes for screws. So initially I fix them to the joist with one screw in the slot of each bracket, slide them up and down until the furring channel is level, then screw them firmly to the joists with additional screws through the holes in the bracket. The Gyprock resilient mount then attaches to the foot of the bracket with either a screw or bolt.
But a purpose built hanger will most likely have several components - a bracket that screws to the joist, a threaded rod that passes through this bracket and fixes with a nut, and another bracket to go on the other end to which the RSIC-1 clips can be screwed or bolted. This will give you fine tuning of the furring channel height over a large area.
If you have a look on page 5 of the PDF referred to above,
http://www.rondo.com.au/rondo/documents/duo.pdf you'll see some 1/4" steel suspension rods and various brackets designed for attaching to joists and to the rod . Also have a look at pg3 of this PDF of the Rondo Walkabout system -
http://www.rondo.com.au/rondo/documents/WlkA2002.pdf - the 129 and 308 furring channel fits the Gyprock Resilient Mount and they use an 8mm (5/16") suspension rod.
This is the sort of gear you are looking for. You're not hanging acoustic tiles nor intend walking on it. So calculate the load (add up your weight per sq/ft - 2 x 5/8" sheetrock plus insulation, lighting etc), work out the spacing of your RSIC-1 clips and the load per hanger, then call your suppliers and ask for an adjustable hanger suitable to carry that load and which has a bracket suitable to attach the RSIC-1 clips.