OpenTK doesn't have direct support of XInput, so you have no way of getting the player number since you don't have access to the right API.
Even with XInput, you would only be able to ask "Does controller 1 exist? If so please give me its state." and not the other way around, that would make it hard to combine with non-xinput controllers, though not impossible.
If you want to put the time and effort into it, you can take a look at XInputDotNet, it is mono compatible and should obsolete the Mid/Vid hack you are using to detect xbox 360 controllers.