I think all the power is wired. Have not had a chance to test it yet, however.
The big change-of-plans is that the electronics will not be powered from the Roomba, at least for the time being. The reason is that the 8-pin DIN serial cable I bought to connect the Roomba to everything else uses 26-UWG wire for each pin, which according to this chart, can safely handle about 0.36 amps. It has 2 power wires in the cable so we can double that to 0.72 amps, but it still isn't enough. The Raspberry Pi plus wifi adapter needs almost that much (about 0.70 amps) and I understand the webcam has comparable needs.
So the fall-back plan is to use a 7.4v LiPo battery from the parts bin. It is connected to the input side of the DC-DC step-down board. The board's adjustment screw is dialed to 5v. The hot and ground terminals of the board's output are each connected to their own screw-terminal jumper strips (each a combination of an 8-position barrier strip and an 8-position jumper strip from Radio Shack).
The RPi is powered by a modified USB Mini-A cable. I cut the host-side (USB-A) connector off, leaving the mini-A connector and a generous amount of cable. I laid bare the individual wires in the cable, leaving the yellow and green wires unconnected. The red (+) and black (ground) wires were connected to the appropriate jumper strips, providing 5v to the RPi.
The RPi is not able to supply enough current to its USB ports to power the webcam. Originally I wanted to hack a powered USB hub (run 5v and ground wires to the wall-wart-adapter plug) but I couldn't find a powered USB hub for cheap. So I hacked a USB extension cable to draw power from the DC-DC step-down board instead. This meant very carefully removing the extension cable's outer insulator (without nicking the inner wires), cutting the red wire, and connecting the red wire to our +5v terminal block such that the webcam gets power. I should not need to do anything with the ground wire -- the webcam will be grounded through the RPi.