That you can't do without PHP as you need to act on the display output of the activity, parse specifically for your activity type, then override its display. The trigger to do that is activity_onDisplayActivity. You'll have to use a Code action with Method set to PHP and under Parameters set variables 1-8 to be references. You can then modify the activity output from your action. The trigger variables are as follows.

&$row, &$title, &$date, &$message, &$insert, &$footer, &$menu, &$extras, $stream, $output

Specifically in your case you'll need to modify $title, which would be done as follows in the code action.

Your stream should be created with the ID of whatever object the stream is in relation to. So if it's an article you'd set the Item or Parent to the ID of that article for example. Next in your code action you'd build the article object from that ID and access whatever information you need from it. Note type, subtype, item, and parent will be gone in CB Activity 4.0.0 replaced by Asset so you'll need to remember to update your code usage when 4.0.0 comes out.