Chris, those are a couple of fine photographs! Anything in that vein, the 645D will fit right in. Of course more action-oriented shooting is possible if you rely more on luck, repetition and timing.
There are examples like the Andy Rouse note from Pål. You won't have the convenience and features of a blazing fast, auto-everything 35mm DSLR but you can always drop back to the D3x for situations where the 645D is going too much against the grain.
The 645D should make a very good showing at A0. I've not done any large scale prints from mine yet but looking at things like the inherent image clarity, shadow tone detail and noise structure, the files blow my Canon files away. The 16x20's I've done are incredibly clean.
While the 645D body is weather sealed, bear in mind many of the vintage lenses -- while optically great -- are not fully sealed. Dust in the imaging chamber could be an issue, though fortunately the 645D does have a sensor-shake dust busting system as one of its modern conveniences.
The 55mm may work for portraiture though it's perhaps a bit on the wide side. The two issues with the 55mm that I've personally seen so far include a lot of strange smearing and bright specks on the innermost surface of the rear element group (looks like a flaw in the final polishing routine during manufacture), and less-than-stellar corner sharpness. If the former issue is corrected, the latter may not be a big problem in portraiture though for my purposes in landscapes and architecture for example it was not really desirable.
I'm looking to the manual focus 67 format 105mm f/2.4 for portraiture, used with the 67-to-645 adapter. This lens is more in the range of what many would consider a portrait focal length, about 82mm in 35mm equivalence. If manual focus doesn't deter you, I think this lens is really well suited to portraiture. Got mine last week and hope to try it out in the near future. Check out Chris Willson's work (Travel 67); he uses the 645D + 67 105mm a lot as a go-to combination. I think he's got several others in use as well, but posts mostly work from the 67 105mm and the new 645 55mm.
The 645 A or FA 120mm f/4 Macro is probably a good choice for portraiture as well, if you can find one. I have an "A" variant coming, just missed the FedEx delivery person this afternoon in fact. Will have it in hand tomorrow and again hope to get in some licks with it before long. Having a full-time day job really cuts into my shooting time...