The half-normal is entirely characterized by one parameter: the standard-deviation of the underlying normal distribution.
One way to tackle your problem could thus be to first establish bounds on the mean of the half-normal using a method as e.g. described in 1. You could then proceed to calculate confidence intervals for quantiles/proportions or whatever you are interested by converting the upper/lower limits of the confidence interval to your measure of interest using standard-equations from 2.
Disclosure: I am not a statistician, if this procedure is wrong, please feel free to correct me.