Part of the problem with Masers instead of lasers would be that the divergence of the beam is larger, so less energy can be focused on the sail.
Hypothetically, for a dielectric reflector for laser light it would likely be several microns thick, since you might want to use several dielectric layers to get a reflectivity that would be higher than a metal film. But would 99% help that much more than 90% ? That would be a system design and economics problem. It might be worth it since you have higher acceleration earlier in the flight. A metal film thin film would probably be cheaper, possibly lighter depending on the materials being compared.
For the Maser - if you have a conductive metal the skin depth will depend on frequency, but at higher frequencies the skin depth gets into the 0.5 to 2 um range, or even less for submillimeter wave.
In general, if the films are highly conductive there should be pretty low losses, probably substantially less than a percent and should get better at low temperatures. You could envision some kind of meta material that would behave similar to a dielectric stack, but it would be complicated, require precision fabrication and not get you much improvement.
I suppose you could coat your maser sail with a superconductor, and if the frequency was low enough it would also have very high reflectivity and could be superior to a normal metal.
The power the maser sail could take would depend on the frequency and conductivity of the material.
Perhaps one improvement would be to try to have an efficient back surface to radiate heat in the infrared red to help manage the thermal effects.
The choice of backing material would influence the thermal transport, but metals are relatively good thermal conductors.
The thermal effect on the modes of vibration and stiffness of the sail due to thermal expansion in large structures could be limiting and require design consideration. Or perhaps a power management plan where you beam less power when the sail is closer to limit bad thermal problems.
