I really thought finding this out would be easier.

When I buy the above listed VTR, can I view my FCP timeline played out over Firewire to an external display while NOT recording, as I can with my old Sony DVCam gear? Which is to say, can I use this to facilitate HD viewing of my timeline on an HD monitor?

Yes the BR-HD50 works as a video pass-thru device.... for SD DV video....
No decks will pass thru HDV video from the timeline. But it isn't so much the decks fault, as the limitations of the format and systems ability to deliver it through FW.
This is one reason why video hardware from AJA, decklink etc have an extra advantage and popularity in HD.

What you can do is pass through the audio, which I do to keep a better quality, and system separate audio channel. I have 2 24" monitors, and setup a custom layout to watch the "canvas" at native pixel resolution on one screen next to a narrow file browser.

For a client, or for review, I'll feed the playback to my 2nd monitor at full screen.

One nice thing about shooting tapes is that you can watch them played back from the deck via HDMI at great quality. It's noticeably better than the component output.

Thanks Sean, that's what I figured but I thought I'd check. So now my decision is to get an IO device before the deck. Just trying to figure out where to place the horse relative to the cart...