I have nothing to offer regarding the code, but you might want to check this thread for information on mrp_structured(): https://discourse.mc-stan.org/t/fitting-a-large-data-hierarchical-model-can-i-trust-variational-inference-algorithm/1780.
Specifically, @bgoodri said (in '17) that it’s available in one of rstanarm’s branches and that it can be installed via:
devtools::install_github("stan-dev/rstanarm", ref = "structured_prior_merge",
args = "--preclean", build_vignettes = FALSE)