There is a spoiler tag which looks like this:
In order to use it you type in this:
[spoiler]Hidden text here[/spoiler]
I think it only allows TEXT. If you're having trouble with it, try removing any punctuation characters or other funny stuff from your hidden text.
the Elf Medium (MV 12", AC 9, HD 1, hp 1/1, AL N) great cloak, lantern
; spells: color spray; scrolls: sleep, sleep, charm person
Hirelings: Georges; torch