When producing a simple geom_path in ggplot2 0.9.3.1 as suggested by the documentation:
df <- data.frame(x = 1:3, y = c(4, 1, 9))
base <- ggplot(df, aes(x, y))
base + geom_path(size = 10, linejoin = "mitre")
This actually produces a bevel, not a mitre (the definitions can be found at this tutorial). I thought this might just be a bug in the documentation, but it seems that linejoin='bevel' and linejoin='mitre' do exactly the same thing.
This sounds like a great feature, but unfortunately we don't currently have the development bandwidth to support it. If you'd like to submit a pull request that implements this feature, please follow the instructions in the development vignette.