> I've updated the default theme's homeTemplate.blogTemplate file and this works just fine but it seems cubersome to modify every theme just in case a user decides to use one of the non-default themes when they view my site.
yes this is the only way to resolve your task. You should remove theme change option (looks like is already removed) and use fixed single theme for everyone. I have removed all themes except my own custom theme and mobile theme and haven't such issued.
> Is there a simple way, using a site-level configuration to add this content for all themes?
no, currently DasBlog don't provide this functionality.