How to serve queued data via BLE?

As peripheral I am supposed to call NRF.updateServices() and set a new value.

Yes, that's right, and you specify notify:true: http://www.espruino.com/Reference#l_NRF_updateServices

But how can I detect if this value was successfully read by the connected central?

Unfortunately I don't think there's any way of doing that at the moment. You could try just calling NRF.updateServices until you get an exception?

My current idea is a multipart API:

I think that's probably the best idea. On newer firmwares the device should have been able to negotiate a higher MTU so you can send 50 bytes each time (not 20) as well, which should make things easier.

I guess on your onWrite event you could have different message types:

• request a packet of data
  
• acknowledge receipt of last packet of data, request a new one
  
• acknowledge receipt of last packet of data and request a new one