[oXygen-user] Load external data into form control?

I'm refining some metadata form controls for a custom framework, and it would be ideal if I was able to load the values and labels for a combobox from an external file (XML or preferably, JSON) rather than hard-coding these values into the CSS. It seems that this may be possible with xpath_eval(). Is anyone aware of any examples of this, or am I heading down a rabbit hole? I'll keep poking at this, but thought I'd see if anyone had thoughts on this effort. Thanks! ...scott