A menu system like in LMA manager (probably seen in other games, this just sticks in my head) would be good.
Along the top of the screen, you have effectively the 'dock' like you're describing. A row of icons that represent all the top level menus, Garage, Dealerships, Race, Tuning, GT Auto, Practice etc.
Along the bottom of the screen, a row of icons that are the sub menus of whichever top level menu is selected, so say if Tuning was selected, you'd have Body, Engine, Intake, Exhaust etc along the bottom.
R1 L1 moves right and left along the top menus, R2 L2 moves right and left along the bottom menus.
Worked so well, and so fast to navigate
If nothing else, GT5 needs a few 'hot keys' that take you straight to certain places. I.e press L3 to bring up the garage, R3 for tuning, that kinda thing...