This should be a basic option. Is this due for the next update? I can't find anything about it. Of course you can edit the templates but the logic doesn't seem to work anywhere aside from the index. Common guys!
Just curious - why would you want that information on forum pages?
I haven't searched for it but there may be a way to add the info to the site footer so it shows on all pages, but presumably this would include an additional query to get the data each time - would it be worth it?
I don't think this is something that needs adding to the default install though - I think it's fine as it is.
When you have hundreds~thousands of users online at all times, it's nice to always have that visible as a social effect. Losing it in forum view isn't good.