That's already the case. The constant entropy condition is the crystal itself, and the state it's in, and ignores all of the equipment necessary to get the crystal into the state where this behavior is possible.
The materials identified for these time crystals aren't very precise oscillators, and the things we use for precise clocks (i.e. atoms and ions) aren't candidates for time crystals. That doesn't preclude some candidate being identified in the future, but for now, and talk of time crystals as clocks is a nonstarter, other than as a novelty item.
Plus, as you note above, reading out the oscillations would require interacting with the crystal, and you'd lose the constant-entropy condition. There are classical analogues to this, where you have master-slave pendulums, so the high-precision pendulum is only perturbed slightly, which minimizes the compromise to its behavior.