I believe uman had a tutorial up at some point on dynamically "loading" entities (including decals) during the level. This would directly relate to your request, since you would be able to, in effect, hide the decal when the player reached a certain distance, and show the decal again when the player was near enough.
I thought it was a sticky at some point, I will try to find it for you now.
[EDIT]
Yup, still a sticky on this board.
Here's the link:
http://forum.thegamecreators.com/?m=forum_view&t=73014&b=23
Hope that helps you
The one and only,