All the CDLC for RS2014 work by pretending to be the DLC for “Smashing Pumpkins - Cherub Rock”, because it was given away for free as a preorder. If you get any of the other delisted DLC, you can use Rocksmith Custom Song Toolkit to change the ID to one you own (free or paid, as long as you have a steam license for it) and it should work.

I didn’t think the project was about independence from Valve, moreso about avoiding duplication of effort among Bottles/Heroic/Lutris and Linux gaming support in general.