Even though I did not overexpose them by at least 2/3rds of a stop, the pictures lacked a lot of details in high key

Can you clairfy that? Particularly the first part.

Yes, a MF camera will capture more total dynamic range than a FF, but light is light. If you underexpose and pull the shadows, you should get more detail than you're describing. The MF will have better color sensitivity but it doesn't appear to be what you are describing. It sounds like microcontrast, which can be affected greatly by lens and PP. Perhaps sensor could have something to do with it, but I cannot see why, especially at low pixel densities (as opposed to P&S which are quite diffraction limited). Do you have a link to the comparison and your own pictures?

Your camera likely has some sort of highlight compensation by default, especially in JPEG. Look at tone curves in DPR reviews, many cameras compress the highlights to capture more overall detail and simulate film. This is why most camera's ISO ratings are optimistic (ie it says 1600 but is more like 1100). I'm not sure if you can completely disable it on D700 and turn it off for JPEG, I know I can on K-x (often criticized for the linear highlights without highlight compensation).

If you are using RAW, you should have complete control. The Hasselblad user could have applied a better tone curve, but it should be within reach as long you capture the extremes of the dynamic range in the high key area you are talking about. After all, you are mapping 4096+ luminosity levels to 256 when you are viewing it on a computer. You may need to post process in some local-contrast.