I believe the updated version of this package covers that case but have not yet used myself: cran.r-project.org/web/packages...
cran.r-project.org
qte: Quantile Treatment Effects
Provides several methods for computing the Quantile Treatment Effect (QTE) and Quantile Treatment Effect on the Treated (QTT). The main cases covered are (i) treatment is randomly assigned, (ii) treat...