The latent heat rule is only forbidden in the engine/power unit section, so other parts could make use of the effect, although most other parts don't requiew cooling. It's given an allowance in the driver cooling section. That said, the brakes can't be liquid cooled, which would limit what phase changing materials you could use there.
5.25.2 Cooling systems: The cooling systems of the power unit, including that of the air destined for
combustion, must not intentionally make use of the latent heat of vaporisation of any fluid
with the exception of fuel for the normal purpose of combustion in the engine as described in
Article 5.11.3.
14.6.1 The driver cooling system is defined as a system, the sole purpose which, is to provide
additional cooling for the driver.
The driver cooling system may make use of the latent heat of vaporisation, or of sublimation,
of a substance.
11.5 Liquid cooling
Liquid cooling of the brakes is forbidden.
As for the cake tins, note the outboard edge (by the spindle) must be sealed/gasketed per the regs, while the inboard edge (by the inboard wheel lip) can be sealed to the wheel or not, it's up to the team. The gap between the cake tin and the wheel barrel looks about 5 mm wide, maybe even narrower. A very thin layer of air then sits between these two parts. Recall Merc used to run knurled/ribbed wheel barrels in exactly that same area a few years ago. It looks like a polymer gasket is used, across teams, on the cake tin photos I've seen, to bridge the wheel to the tin. Either way, the choice will determine whether or not the thin sleeve of air between the cake tin & the wheel is a sealed envelope or not. Maybe they want it to be sealed so that it's acting as a more stable thermal pathway between the two parts. The "cake tin"/drum itself is such a thin, lightweight part that it's hard to image it having much influence as a thermal mass (using phase change material or not) upon the greater mass of the magnesium wheel.
3.14.2 Drum
The Front Drum and Rear Drum bodywork:
a. must be made to the geometry defined by RV−FWH−DRUM and RV−RWH−DRUM
respectively.
b. must be fitted with an Aerodynamic Seal, in the outboard of the two annotated
volumes, between the drum and the axle.
c. may be fitted with an Aerodynamic Seal, in the inboard of the two annoted volumes,
between the drum and the wheel rim.
d. both Aerodynamic Seals must be circumferential, continuous (around an arc of 360°)
and uniform.
It could contribute some or the ducts could be no different in terms of losses to any other team. Difficult to say without measurements.
