I don’t see that as a good idea. The 12-speed Eagle chain is more narrow than the 11-speed chain, and the cassette cog width is similarly narrowed. Putting a 12-speed chain on a wider 11-speed cog may lead to issues.
See the following for related info:
- Cassette cog spacing is 3.8mm on their 11-speed groups, but that shrinks to 3.65mm for Eagle’s 12 speed (with a slight bit more room between the smallest two cogs to accommodate the angle of the chain there). Chain width goes from 5.6mm on 11 speed to 5.25mm with 12 speed. The change in width isn’t really even important, it’s everything else they did to make it the best, strongest, quietest and smoothest chain. Ever.