For an unbalanced panel data set, one may consider padding NaNs in the response variables for those cross-sections with fewer observations in the time dimension. For example, at j=1 there are 2 observations, at j=2 there is only one observation. By artificially creating a second equation with fake regressors but NaN in the response variable at j=2, an unbalance panel becomes a balanced one. MVREGRESS uses Expectation-Maximization (EM) to maximize the log likelihood function. The EM algorithm is friendly to missing values. I think RVREGRESS will work as usual in the presence of NaNs.