This is not possible...
For starters, scripts do not have access and don't control or handle what is shown in the contact list (or desktop contact windows).
Second, what you suggest is a very extremely high security risk. URLs should not be made 'invisble'. In that way the user (or anyone, even yourself) can accidently click on a link which says it is 'MySpace' but actually links to a virus download. It has been requested before, but for this sole reason it will never be implemented.
What I can suggest is to use the 'Notes' feature in Windows Live Messenger. For this you right click a contact > Edit Contact > Notes.
You can also find a 'website' field in the 'Personal' tab.
PS: BBCode is not enabled as-is. What you refer to are formatting codes used by Plus!. This is not really 'BBCode'. These are just a series of characters which happen to be the same as the formatting codes used in BBCode. Aka, it is not like Plus! enabled BBCode to be used in screen names, it is just that Plus! now also uses the some formatting codes which are also used in BBCode.
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