This manual can help you:

https://mcnp.lanl.gov/pdf_files/la-ur-17-29981.pdf

FMESH defines the mesh superimposed over the geometry, that means independent from the geometry definition, and so provides no access to the properties of the underlying geometry like the material number.

I was able to come up with the following solution:

1) insert one isotope with small cross section and in small concentration in each of the materials;

2) define dummy materials containing only those isotopes;

3) define an FM card related to the FMESH (one for each dummy isotope) which will apply a multiplier to what you're computing (in my case, the dose) only when the particle finds the dummy isotope.

The effect is that the meshtal will have a value different from 0 where the dummy isotope is hit and 0 otherwise.