i2c gpio expander on hotplug i2c-adapter

Hi there,
I'm a bit puzzled. My desired setup is something like this: Linux station => usb i2c-adapter (i2c tiny usb) => i2c switch (pca9548) => i2c gpio expander (pca9555)
So far, instantiation works for the i2c switch using the sysfs method (echo pca9548 0x70 > /sys/bus/i2c....). What's really causing me troubles is instantiating the pca9555. What I found so far indicates that it has to be defined in the machine specific file. So that is what I already added:

static struct pca953x_platform_data pca9555_0 = {
    .gpio_base = 32,
};

static struct i2c_board_info __initdata i2c_tiny_board_info[] = {

  {
    I2C_BOARD_INFO("pca9555",0x21),
    .platform_data = &pca9555_0,

}
};

Now I'm struggling how to declare my i2c tiny usb adapter to be the bus where the pca9555 should live on (leaving out the i2c switch for the moment). What I tried so far did not work (obviously):

static struct platform_device i2c_tiny_usb_device = {
.name = "i2c-tiny-usb",
.id = 0,
};
static void __devinit i2c_tiny_usb_init(void) {
i2c_register_board_info(0,i2c_tiny_board_info,
ARRAY_SIZE(i2c_tiny_board_info));
platform_device_register(&i2c_tiny_usb_device);
};

The i2c_tiny_usb_init() funciton gets called in the init section. It compiles without errors, but with this kernel, the i2c-adapter now gets registered as i2c-1. How would a proper declaration of that i2c-adapter look like? Or would there even be a chance to do that instantiation from a userspace program? I saw that the symbol "pca9555_0" is listed in the system.map file of that kernel. So, would there even be a chance to just declare the gpio platform data in the kernel and do the instantiation in userspace?
Thanks in advance,

Hartmut