I think I looked at this in the past. What happens when the field contains HTML? When the XML response is converted to JSON does the HTML get converted to JSON in a nested fashion? I don't think you would want this behavior if it does and that has been a concern with some other libraries I have looked at.
The problem really has to be solved by QuickBase. The existing XML flat format has a lot of warty behavior (and XML structured contains too much query and field filler) - for example [%] and [-] both convert to <_> as XML and then what does the X2JS do with the duplicate _ property (it gets overwritten)? In general XML has greater expressiblity than JSON because XML is ordered and can contains duplicates while JSON is neither. And XML does have a benefit that it can be search with CSS selectors while JSON cannot be so easily searched (there is no selector language like XPath or CSS for JSON).
It is great to see someone else looking at this with fresh eyes. I am going to take a second look at this library myself.