[R] lattice: how to "center" a subtitle?

[Marius Hofert]

Dear expeRts,

I recently asked for a real "centered" title (see, e.g., http://tolstoy.newcastle.edu.au/R/e13/help/11/01/0135.html). 
A nice solution (from Deepayan Sarkar) is to use "xlab.top" instead of "main":

library(lattice)
trellis.device("pdf") 
print(xyplot(0 ~ 0, xlab.top = "This title is now 'centered' for the human's eye", sub = "but subtitles are not centered", scales = list(alternating = c(1,1), tck = c(1,0)))) 
dev.off()

My question is whether there is something similar for *sub*titles [so something like "xlab.bottom"]? As you can see from the plot, the subtitle does not seem to be "centered" for the human's eye. I would like to center it according to the x-axis label. 

Cheers,

Marius