The only way I know of is to compute the box values yourself like this:
library(plyr)
xx <- ddply(mtcars,.(cyl),
transform,
ymin = min(mpg),
ymax = max(mpg),
middle = median(mpg),
lower = quantile(mpg,0.25),
upper = quantile(mpg,0.75))
ggplot(data = xx,aes(x = factor(cyl))) +
geom_boxplot(aes(ymin = ymin,ymax = ymax,middle = middle,upper = upper,lower= lower),
stat = 'identity')
There are some warnings on the ddply
call, but you should be able to ignore them safely.