I want to grab some disk images (taken with dd from a raid5 array) and re-assemble them. The idea is that I slap the files back together and then offer the data as a device, like /dev/virtualraid or something like it.

I don't think you can (easily) do that. Raid 5 isn't just simple striping - and it happens on a pretty low level. Assuming we aren't talking about hardware raid 5, you'd need the meta information of your volume manager, and mimic whatever your volume manager does.