It's just training the source association together with the training set:

https://github.com/mukhal/intrinsic-source-citation

The only 2 reasons big LLM providers refuse to do it is

1) to prevent a long slew of content creators filing class action suit.

2) to keep regulators in the dark of how feasible and actionable it would be, once regulators are aware they can perform the source-aware training themselves