Dear Jonathan,
Studying the question of connection of entropy and gravitation, I found Lorentz-invariant formula for entropy in the book: Fizika i filosofiia podobiia ot preonov do metagalaktik. Perm, 1999, 544 pages. ISBN 5-8131-0012-1. In short the question is described in the book: The physical theories and infinite nesting of matter. Perm, 2009-2012, 858 pages. ISBN 978-5-9901951-1-0 in such way: Using the stress-energy tensors for the substance and the gravitational and electromagnetic fields allows us to write the equations of thermodynamics explicitly in the Lorentz-invariant form. As a result the entropy, the amount of heat, the chemical potential, the work and thermodynamic potentials can be represented as tensor functions of microscopic quantities, including the electric and gravitational field strengths, the pressure and the compression function. This allows us in § 21 to find out the meaning of the entropy as the function of the system state - it is proportional to the ratio, taken with the negative sign, of the absolute value of the ordered energy in the system to the heat energy, which is chaotic by nature. The ordered energy means the energy of directed motion of the substance, the compression energy from pressure and the potential energy of the substance in the gravitational and electromagnetic fields. When the system achieves equilibrium, part of the orderly energy inevitably is converted into thermal form and the entropy obtains a positive increment. I hope it may be interesting for you.