Karel,
Yes, you can perform population-level analysis on surface in AFNI. Could you describe the types of those explanatory variables (including covariates)? Are they between-subject or within-subject?
> Is it possible to do something similar as ETAC does in 3dLMEr's output (also considering that I am using surfaces as dependent variable)?
3dLMEr is based on linear mixed-effects modeling at the voxel level, and does not offer a way to adjust for multiplicity.
Gang