Create Node using JSON and Multivalue field, Auto-complete Widget FAILS

Dear all,

After questioning google the whole day, I am eventually opening this issue...

I'm trying to create a node (endpoint, create node) consisting of, among others,
the following structure:

field_citcomponent_devref (Array, 17 elements)
  und (Array, 25 elements)
     0 (Array, 11 elements)
     1 (Array, 11 elements)
         uid (Array, 36 elements)

Usually the field is filled via the user_reference_autocomplete widget in the UI
which works well, also for 10+ user references. However, I am able to fill the _FIRST_ value
using the node create endpoint with this JSON syntax:

"field_citcomponent_devref": {
       "und": [ {"uid":"my_user_name"} ]
   }

I tried several mappings of the Drupal array structure but nothing is working, here
are some examples (not working):

"field_citcomponent_devref": {
       "und": [ {"uid":"my_user_name"}, {"uid":"other_user_name"} ]
   }

"field_citcomponent_devref": {
       "und": [ "0": {"uid":"my_user_name"}, "1": {"uid":"other_user_name"} ]
   }

Can anybody tell me why the syntax of the JSON string passed via POST is completely
different depending on the widget the actual form makes use of?
More importantly, has anyone managed to add multiple references to a multi-value field
which uses the auto-complete widget?
I also need to do this with node references...