Yeah, when you have the Account in your Contact list the Pidgin plugin should hande this.
I thought you talked about finding new contacts - the process of checking if anyone in your Contacts has WhatsApp. I dunno how this works but if it usually would send the whole contact list of you to the server I thought it may would be better when we would be able to integrate some options so that not the whole adress book is sent to WhatsApp but instead only the Numbers you want to check

Edit: But I think the most important at the moment is that we find somebody who knows how XAMPP works and who can Programm the (Pidgin) plugin for the N900 (and maybe also for the N9)... - I failed at trying to do something like this