Hi Jimi,
Comparisons between Zen and Vipassana are tricky. Having participated in one zen practice period in a Soto Zen temple (here in the US), I recognize some of your questions. I don't have the expertise to provide a thorough comparison between the Theravada and the Mahayana (of which Zen is a part). My time with Zen was brief, but remains very important to how I understand practice--still, it hardly qualifies me as a Zen expert....anyway, here goes:
is zazen really just samatha practice with the object of meditation being merely "the present moment" rather than "the breath"?
In a word: kinda. Zen practice is by and large a concentration practice, so it does lead to concentration states which can include experiences that the Theravada would attribute to jhana. But the important thing here is that zen practice goes far beyond zazen, into everything you do, your interactions with people, cleaning, eating....etc. Ideally, Theravada practice does too, but it's a bit more free-style. What I'm trying to say is that concentration states can be cultivated (or discovered as it was in my case) in formal and not so formal activities like bowing and prostrations,
soji (temple cleaning...ie: cleaning bathrooms, sweeping floors). Zen directs that when you sweep the floor you leave yourself behind so that there is "just sweeping," that when you do a prostration, you "become the prostration." Any resentment, joy, etc. that develops are to be set aside in your concentration on sweeping--similar to becoming "one with the breath." Vipassana asks the practitioner (at some point) to investigate joy and resentment, or to investigate jhana itself.
Both practices advocate continuous awareness, but do so in very different ways.
You didn't ask for it, but from my perspective, I don't know how effectively you can study zen without really throwing yourself into that life, ie. moving into a zen community. Just showing up for zazen didn't seem to be enough, at least so it was for me. (This may apply only to Soto Zen, since I have no experience with Rinzai).
Below is a link for a very interesting talk contrasting Zen and Vipassana by Gil Fronsdal, who ordained as a zen priest, but eventually went very deeply into the Theravada. (Use the "search talks" function to find "Zen and Vipassana" given on 2007-12-09). It helped me a lot as I was trying to untangle these differences. But the bottom line is that it's hard to compare and contrast these practices.
http://www.audiodharma.org/talks/?search=zen
Hope this helps!
Regards,
Bruno