The ballnuts from China are made from a fairly hard grade of steel. However I did check a RM1204 ballnut from Chai and I could file it, so you should be able to machine them.

There are some suppliers on AliExpress with small ballscrews - it might be worth asking if they have what you need although it's a bit of a long shot.

Could you not mount the ballscrew somewhere else, for instance to the front of the table perhaps using the convenient T-slot? It's clearly not as nice as having it under the table, since the force will be off centre and you'll need to protect it from swarf, but I don't think either of these are a big deal compared to what might happen if you mill that much out of the table. It wouldn't require modification to the machine in addition to being a lot cheaper.