ggplot.multistats currently provides stat_summaries_hex and some helpers.
stat_summaries_hex is similar to ggplot2::stat_summary_hex, but allows specifying multiple stats using the funs parameter (see Example).
ggplot.multistats is not yet on CRAN. Install it using the devtools package from GitHub:
Specify a summary variable using the z aesthetic and specify a list of funs to provide stats for you:
library(ggplot2)
library(ggplot.multistats)
ggplot(iris, aes(Sepal.Width, Sepal.Length)) +
stat_summaries_hex(
aes(z = Petal.Width, fill = stat(median), alpha = stat(n)),
funs = c('median', n = 'length'),
bins = 5
)