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I think the trick is to put parens around just 'less'.
That way, 'less' is executed in a subshell, and the
first shell directs the stderr of the subshell to /dev/null.

Works for me on sunos5 and linux.

This behaviour all makes logical sense to me now --
it can all be explained by supposing that bash
directs any error messages to the stderr of the
command, whereas other kinds of sh direct error
messages to their own stderr.  Using a subshell
should make it quiet for everybody.

But then didn't have to use parens to get it to be
quiet in Linux (like Neal) -- so i don't know what's
different about our setups.