out of the box, afraid not… just like wp, sp treats each multisite install as an island with no sharing of data…

but you could write a small plugin that would run on the main site that could pull the data from the db and display it… its on our list to see if we can add such a plugin some day, just not sure when…

Is there any code in the ‘Template tags and Widgets’ plugin that can be reused for this?

There is certainly code in the template tags plugin that could be re-used for the display side of things. The display is not really the problem. The main issue that needs addressing is the database connection that is always geared to the current WP instance. Any code written has to address it’s queries to a different set of tables. Again – not difficult as such but not built in.

Another issue that would need designing and coding is that of permissions. Who on the remote site has access to what forum data on the other. It might be that in your case everyone does but if and when we create something like this it would need to be taken in to account.

I may be proposing something that is more trouble than it is worth but it might be worth looking at an RSS feed to draw in the SP information and then working some code to display that. Maybe worth thinking about anyway.