the described method should work, but don't forget to set "spawn at start" to no, so the decal can only spawn when triggered. This is from the top of my head, will check and edit later if I'm wrong.
EDIT: forget about it, decals have no "spawn" property, bummer.
EDIT2:Now here's what you do!
place your decal and in it's property panel, change the following:
1.Give it a unique name (i named it decal for simplicity here)
2.in its "Start" property, give it the script:"appearifactivated.fpi" from the scriptbank.
place a triggerzone, in its "Main" property, change the script to "plrinzoneactivateused.fpi"
in its ifUsed field, put in the name of the decal.
Test: walk into your trigger zone and see the decal being spawned.
I am Jacks complete lack of surprise.