> Any thoughts?

I got an idea of how to solve this problem in an OS-independent manner. The USB
devices and hubs form a tree, like this

  Root  HUB
  0   1   2
  |   |   \__...
  |   \___
 DevY     \
         HUB
        0 1 2
        |   |
        |  DevX
       HUB
      0 1 2
          |
        DevZ

This tree can be considered as an ordered tree, if you read it from left to right.
In that order, the devices are orderd

DevY - DevZ - DevX

Since the devices are ordered, the "instance" parameter from musb_init can be used
to identify them uniquely, like

instance==0   => DevY
instance==1   => DevZ
instance==2   => DevX

So I would say that we can use the current API using the "instance" parameter to
uniquely access a device. All we have to do is to build that tree, sort it, and then
use the instance parameter as an entry to that tree. The sorting takes care of
different ordering, which can happen during enumeration (depeding on power-up
sequence, phase of the Moon etc.). So if you have three devices like above, DevZ
should alway be at "instance==1". The only problem is if you unplug DevY for
example, then you get the map

instance==0   => DevZ
instance==1   => DevX

which is different from above. But if you have a different number of devices, you
likely have to change your frontend cody anyhow, so you can change the device
mapping there as well. 

In order to simplify the code, I would not build a complete tree and sort it, but
scan the whole tree hierarchically, i.e. look at

Bus1/Port1
Bus1/Port2
Bus1/...
Bus2/Port1
Bus2/Port2
...

Since there is a maximum of toal 127 USB devices, this scan should be pretty quick.
If you find a device with matching vendor and product ID, you increment an internal
counter. If that counter matches your instance parameter, you open that device.

The ultimate solution of course is to put an additional address into each device, so
you can distinguish them easily. For a out-of-the box Web cam you probably have no
chance, but for the home-made MSCB nodes I put such an address into each node, so I
can distinguish them even if the have the same product and vendor ID.