Does SilverStripe contain a way to add markup from a template to another template? Symfony calls it [url="http://www.symfony-project.org/gentle-introduction/1_4/en/07-Inside-the-View-Layer#chapter_07_sub_slots"]template slots[/url], and Ruby on Rails [url="http://api.rubyonrails.org/classes/ActionView/Helpers/CaptureHelper.html#method-i-content_for"]content_for[/url].
I had a look at the reference pages of doc.silverstripe.com and the Sapphire API, and couldn't find an answer. I'm also quite new to SilverStripe, so I don't fully understand the capabilities / limitations of SSViewer yet...
I don't *think* there is anything to support that sort of dynamic injection yet. The current 2.4 SSViewer is little more than a simply string search and replace and doesn't really have any sort of smarts, hence using PHP and renderWith() which you can overload on your controller actions etc.
SSViewer is getting a bit old overhaul for 3.0 so implementing functions like that may become easier.