It could be added as option, the administrator will be able to define the default mood for his users.
I believe many users will not notice this addon as he needs to open user menu and open the mood page to select their first mood. But if the user find a default mood icon below his name everwhere then he will be encouraged to use this addon and click on the default mood icon to choose.
Do you get my point? The same idea and concept was used in XF1 addon as well.