Earlier quoted context omitted.
There was a pretty successful Kickstarter a while back for a product ("Coffee Joulies") that was exactly that, but for your coffee mug. Little bean-shapes made of stainless steel, filled with material that changed phase at 140F.
There's also Ember [1] (also a kickstarter I think) that just made a mug with a heating element and controller to keep your beverage at a specific temperature. Using materials with specific phase change temperatures sounds like an innovative solution, but it's also grossly engineering a problem that can be solved better by $10 worth of components. [1] https://ember.com/products/ceramic-mug
I'm not sure I can agree about a phase-change solution being overengineered, however; while the materials would be more complex, the design itself should be far simpler than an electronic mug, and exactly the same as a vacuum sealed one.