Basically what it would be is a player status that makes you not pop up on the in-game player search. Simple as that. It's not Busy. As busy blocks out all communication besides local chat and linkshells.
An Incognito mode could help against RMT, FC invite spam, and stalkers who just want to get on your nerves by following you. While allowing your LS, Friends, FC mates, and anyone in proximity of you to still /tell, invite to group, whatever