Hmm...
I forced gnash to open the URL received in the play() command [I still don't know why I have to do this], and it still didn't work.
Eventually it turned out that minor obvious changes to the script on the wiki did in fact work, but just needed a longer timeout than the default.
Example working script, plus the added parameter to ask for high quality version (MP4:AVC/AAC-HE) below.
url="" href="http://www.youtube.com/watch?v=CTpaYffP8PU">http://www.youtube.com/watch?v=CTpaYffP8PU
vars=$(wget --quiet -O - "$url" | grep -F watch_fullscreen | cut -d \? -f 2 | cut -d \" -f1)
echo "$vars"
gnash "
http://s.ytimg.com/yt/swf/watch-vfl51838.swf?$vars&ap=%2526fmt%3D18"
Cheers, s