Just read this complete thread from page 1, 2 cups of tea later I am at page 47 :-)
Thats some epic commitment!
The reason they have to be the exact rings is that the approach I took was to drive the LED's nativeley via the NUD4001 LED driver chip. Its what all modern Audi/BMW etc use to run the LED's in them.
That chip is essentially a micro buck/boost Power supply that guarantees a perfect flow of current to the rings. The amount of current flowing is governed by a specific resitior value on the controller. More or less resistance = more or less power. The rings we use require about 400mA of constant current to operate. So as long as the ring you choose to use has about the same power requirement it will be fine.
You can figure out how much power a ring needs by putting a multi-meter inline with it.
Also, rings that have additional resitors, or extra black boxes with power supplies are no good unless they can be removed. The NUDs are designbed to drive the LED chain directly not via any other electronics.
Furthermore, the NUD's are never actually on permenantly! They are flashing on and off at many thousand times per second. This is how we change the brightness, by altering the frequency. So again, anything between the NUD and the LED and it wont cope with the on/off duty cycle.
So in short, yes you can use different rings, but only if they are direct @ 400mA so if you have some knocking around, check the power they draw to see if they are suitable.