@aral >We need a free data license that stipulates that “if you use this data, your code must be released under so-so-and-so license”.
Unfortunately, there is no way to do that with copyright law, as merely using data doesn't require copyright permission.

It is possible to license data under the AGPLv3-or-later, provided there exists a source form of the data.

Really, it you distribute the data incorporated with a AGPLv3-or-later program that does operations on the data, chances are that proprietary interests wouldn't go through the effort of extracting the data out.