I could have done with a script like this just now. I was helping someone out with just this problem, but I couldn't find nullfilter, and Dan Bernstein's stuff is a bit, um, legacy, in the build process and I didn't hold out much hope of it working on MacOSX. In the end I used script, not least because the intermingling of stdout, stderr, and the tty is actually what we wanted (for once).

Strange how you read something thinking I've never wanted one of those "all these years", and then within a month or so you do! :)