bb code modification

[rin]

hello,

here's one for you. i'd like to have a bb-code which works like this:

[noguest]this text won't appear to guests even if they are allowed to view a thread[/noguest]

i think the bb code speaks for itself. the idea is to have a bb code which hides text or pictures and anything within the text to guests. i think thats not possible with just adding a complicated bb-code in the bb-code manager.
i hope that someone else also finds this useful and is encouraged to make a modification for it. if it is possible without modifying any code or if there already is a hack, please tell me where to find it. thanks a lot.

ps: would be cool instead of displaying nothing to guests showing text telling them to register to view the hidden message.