Thought I’d noted this before. I was very happy when I learned of its existence.
Cmd + Option + C will copy the selected region and place it into the source monitor. You can then treat that sequence of clips like you would anything else in the source monitor. Great fun.
Similarly the Lift and Extract commands can be modified with Option.
Option + Z will lift the selected region and place it in the source monitor, while Option + X will extract and place in the source monitor.
Update 2013/07/11 Here’s me learning all kinds of basic keyboard shortcuts again. In Media Composer, C is your standard “Copy to Clipboard” shortcut, so Option + C is the modified shortcut you’re after here.