evaluated location in assign scxml tag

Question

how to dynamically specify location in assign scxml tag? My scxml interpreter does not accept something like this. Parametr expr="_event.data.expr" is evaluated properly. But location parametr accept only exact name string.

<scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0" binding="early" xmlns:qt="http://www.qt.io/2015/02/scxml-ext" datamodel="ecmascript" name="StateMachineMain" qt:editorversion="4.3.1">
...
<transition type="external" event="_assigndata">
    <assign location="_event.data.data_id" expr="_event.data.expr"/>
</transition>
...

For now I'm going to use script bound to event to set the value, but it's somehow clumsy