Unfortunately, the answer most probably is: no, (currently) not possible. In order to sync, you need to communicate the time in some form to the camera, and to run a routine on the camera that makes the adjustment. It is the communication part that presents a problem. It is easily possible to detect the +5 signal on the USB bus and trigger some event accordingly (such as to take a shot), which is what all the "USB remote" story is about. But that's it. Well, I believe somebody tried, with limited success, to use this for a (slow, one-way) serial communication to the camera, which would probably suit your case, but there is nothing ready-made. You will have to write the code, on both ends, by yourself.
Else, PTP (Picture Transfer Protocol) as such does not provide for clock sync, as far as I am aware. Well, Canon's PTP implementation contains extensions (such as a possibility of remote capture), but they are undocumented of course. Finally, Canon implements a proprietary DCP protocol, which presumably gives full control, but it is totally unexplored. There is even nothing to reverse-engineer, as there is no available software that uses the protocol.
