Cannot set custom descriptors

Please can you explain a bit more about what you want? What additional metadata do you want to include? Apart from the text description (which is implemented, but it seems nobody really uses) I wasn't aware of much other metadata that could stored for characteristics.

What sample rate do you want to change? Is there a problem with just sending a command over the Nordic UART characteristics?

There doesn't seem to be a way to create a custom descriptor that holds a specific value that I would specify, it's in neither NRF.setServices or NRF.updateServices

Is the problem that you want to use your own UUID in setServices, not one of the 16 bit pre-defined ones? If so, https://www.espruino.com/About+Bluetooth­+LE might help, but you can just make a random number like c28bef9ab6ea685a3c742eb46418eb9e

Then change the second set of 4 digits:

NRF.setServices({
  "c28b0001-b6ea-685a-3c74-2eb46418eb9e" : {
    "c28b0002-b6ea-685a-3c74-2eb46418eb9e" : {
      value : "My custom data",
      readable : true
    }
  }
});