transparency in raster images

I've just been hacking image.SpatialGridDataFrame to support
transparency, either via specifying an alpha band or a constant value
of alpha:

image(foo,red=1,green=2,blue=3,alpha="band4",add=TRUE)
image(foo,red=1,green=2,blue=3,alpha=0.5,add=TRUE)

I don't think what I've done works properly with NAs in the data
though. Has anyone else implemented transparency for SGDFs to save me
spending time fixing little awkward bits like that? Roger?

Barry