  The sensitivity of ALMA makes it possible to detect thermal mm/submm emission
from small/distant Solar System bodies at the sub-mJy level. Measured fluxes
are primarily sensitive to the objects' diameters, but deriving precise sizes
is somewhat hampered by the uncertain effective emissivity at these
wavelengths. Following Brown and Butler (2017) who presented ALMA data for four
binary TNOs, we report ALMA 1.29 mm measurements of four Centaurs (2002
GZ$_{32}$, Bienor, Chiron, Chariklo) and two TNOs (Huya and Makemake), sampling
a range of size, albedo and composition. These thermal fluxes are combined with
mid/far-infrared fluxes to derive the relative emissivity at radio (mm/submm)
wavelengths, using NEATM and thermophysical models. We reassess earlier thermal
measurements of these and other objects -- including Pluto/Charon and Varuna --
exploring effects due to non-spherical shape and varying apparent pole
orientation, and show that those can be key for reconciling previous diameter
determinations and correctly estimating the spectral emissivities. We also
evaluate the possible contribution to thermal fluxes of established (Chariklo)
or claimed (Chiron) ring systems. As a general conclusion, all the objects,
except Makemake, have radio emissivities significantly lower than unity.
Although the emissivity values show diversity, we do not find any significant
trend with physical parameters such as diameter, composition, beaming factor,
albedo, or color, but we suggest that the emissivity could be correlated with
grain size. The mean relative radio emissivity is found to be 0.70$\pm$0.13, a
value that we recommend for the analysis of further mm/submm data.
