Extract parts of a matrix parameter

My model has a matrix as parameter. Is it possible to only extract the posterior draws of a specific row or column, without having to extract the whole matrix first? I can also extract a specific element:

fit$summary("matrix_param[1,2]", "mean", "sd")

But that is not what I want…thanks.

Check out the rvars format from the posterior package (now on CRAN).

I am sorry but I fail to see how rvars format helps. Can you please provide a specific example? Thanks.