This is so close to being a general purpose data retrieval function that I think it's worth going all the way. How about this idea:

* `registerDataObj` loses `dataFor` argument and instead gains a `filterFunc` argument
* The implementation of `registerDataObj` saves the filter function (`list(data=data, filter=filterFunc)`)
* The URL no longer has datatable or selectize in it, but rather, always "dataobj" or whatever
* Line 512 and 522-523 instead of switching on matches[2] just pulls out the filter func from the download object and use that

It would be even nicer if the filterFunc wasn't responsible for parsing the query string and formatting the results as JSON.