I sent an email to Festool regarding this, and got the following reply (in 2008):"thank you very much for sending us your idea regarding an new sander for the CMS system. For the moment we have not planned starting activities for making new products for the CMS product group. If we have the capacity to start new products in this segment we will put up your idea an discuss it.
Please don’t hesitate to send us your ideas. If we have the possibility and see a potential for the idea we do our best to realize it."
I don't think there's much chance of one being developed.
I did however find an ancient Festo Basis 3 module, which is a disc/spindle sander module for the Basis and CMS system. For my needs, it's much better than the CMB120 band sander module.
Now all I need is a dust collection accessory for the spindle sander:
But I've only seen this once or twice on ebay.de in the last couple of years, so I guess I'll have to make my own.