Bidirectional io.open()...anyone got one?

[Duck <duck@...>]

I suspect that this is a perennial question (judging by a quick look at 
some list postings of the past)...
...but does anyone have either a patch or a package which implements a 
bidirectional popen()? One which allows not just "r" xor "w" like the 
standard version, but also "rw" so that I can use the file handle it 
returns to push data into and get data out of the stdin/stdout of the 
subprocess?
Oh, and which is portable between at least Linux and Windows, if not also 
OS X and the BSDs?