For signing in offline, please refer to this post:
CookieRevised's reply to how to sign in on appear offline?
(and both MSN Proxy and MessengerDiscovery let you choose your status, if you want another signin status then Offline. Also SatusSaver can do it in a way...)
For Polygamy, please refer to this post:
CookieRevised's reply to Polygamy (what/why/when/etc...)
In MSN Messenger 7, you will be able to choose you sign in status, (making the "sign-in offline"-plugins/addons useless)
/ 
