Redcat,
The letter from Mitsu is spot-on, where they don't really recommend going for the "field fix" (which is fitting the brackets). The reason is that the strip/assembly of the dash will probably leave you with more (internal)rattles. I think that is my situation - and if the dash is really shaking less, it isn't by much.
Will you be getting the brackets under maint plan? I suppose not, as the GLX is only covered by a service plan.
Here is my recommendation: get the brackets fitted, but try to put pressure on the tech to take his time and to do a carefull job of re-assembling. I don't know how you will achieve that, but if you fail the car will be worse.
I think I have given up. I tried to trade mine in on a new Treg - only offered me R210k (2008 with 80k on the clock), so I refused to continue with discussions.
After our mountain trip this past weekend, have I not even bothered to wash that handsome red beast of mine - I just couldn't be bothered. When we went up a badly rutted steep zigzag climb did I not even try for the best line, I just kept my foot steady and left the fancy driving to the T/C computer. In other words, for the first time in my life am I feeling buggerall for my car.
I'm just going to see where this whole thing is taking me, for the rest am I enjoying the good sound system a to drone out the rattles (I'm luckely from the hard rock era, so cranking up the volume is just fine

)
If you can find a tech that can strip the dash, fix joints and stuff with blackmagic and get the rattles to go away, I promise you I will drive to him (anywhere in the country) and allow him to fix mine as well. I won't say "at any cost", but am almost saying it.