library(rms)
exp.var <- names(iris[,c(2:3)])

ddist <- datadist(iris); options(datadist="ddist")

for (exp in exp.var) {
   mod <- ols(formula(paste("Sepal.Length ~ Petal.Width + ", exp, sep="")), data=iris)
   #p.mod <- Predict(mod, name=exp)
   summary(mod, exp=quantile(iris[,exp], probs=c(0.2, 0.8)))
}

Error in summary.rms(mod, exp = quantile(iris[, exp], probs = c(0.2, 0.8))):
 factor name(s) not in the design: exp

summary(mod, Petal.Length=quantile(iris[,exp], probs=c(0.2, 0.8)))