I don't see any way to get AE to display the embedded timecode of a source video. AE does know this, as you can see it by looking at the metadata for a video.
Does anybody know a plugin that does this, or has anyone written an expression already to do it, using the timecode window effect?
After Effects doesn't take into account the embedded timecode in video files.
The Timecode effect (Effect > Text > Timecode) will overlay a timecode readout, but you have to set the starting frame. Again, it doesn't get the embedded timecode.
As explained, AE does not expose this stuff. not to plug-ins, not to expressions. It's a long-standing issue, but can't be helped. In addition to using the TC effect, it may alos help to set the composition start time to the in-point time of the clip. Just a thought: Another way would be to take a detour via Premiere - apply the TC effect set to Media as source, import the Premiere stuff as DynamicLink items.