I've written a kind of tutorial listing these undocumented commands I heard about, and has also been completed by other scripters, while adding coding example to these commands but it will be of no use here as it's written in French.
Well that was actually posted by Soul_Eater, not me. But anyway, if /setlayer isn't working for you it's probably because you're on Windows 95/98/ME. Transparency is only available on Windows 2000/XP.