It's not obvious to new players how special the SMC is. Thus many tend to die there. We should add special messages (of increasing frequency about the increasing deadliness of the cave).
I think your other suggestion, if implemented would be more than enough, combined with the already existing scroll of warning: http://www.adom.de/forums/project.php?issueid=5372 But dying in the small cave is probably one of the lesser problems for new players anyway. They don't probably lose much progress before discovering that they should be careful with it. And learning by dying really is part of the game, I don't think that aspect of roguelikes should be toned down too much.
But they are killed in a rather unfair way and seem to learn "You get killed without chance in an early dungeon" (which kills fun) instead of "You get killed in a chilling and somehow special early dungeon". I don't want to tone down the killing but I want them to learn the correct things :-)
Isn't there already the scroll of warning? Although you need literacy to read it.
Literacy is one point but as one rather experienced roguelike player told me "I'm not going to read scrolls just for fun... who knows how they will screw me...". I probably should have implemented post signs with messages instead of a scroll ;-)