The Canon engineers are probably correct in saying that the PC port is a low-voltage port. I would not recommend putting much current at all through that port.

There are a couple of ways of solving your problem:

• Add a single transistor on the input to your flash. Then, your flash trigger power will become negligible, because the transistor will be triggered through the camera, and all the high-current flow will be within the device itself.

• Use the hotshoe. The center pin of the hotshoe can be used to trigger most flashes. I can't say with certainty what Canon uses for the hotshoe mount in the 5D Mark III, either, but according to a discussion thread on Electronics point, the XT uses a 2SK2715 MOSFET for the flash trigger, and it's probably a good bet that they use the same part on all their cameras. The 2SK2715 is made by ROHM Semiconductor, and according to the spec sheet, it is rated at 500V at up to 2A. Canon downrated their maximum spec to 250V, either as a hedge against slightly substandard parts or to allow them to source other equivalent parts that may have looser tolerances. And, of course, a part designed to handle up to 1 kilowatt triggering power laughs at your 0.3395 Watt triggering current requirements, and any even-semi-plausible replacement MOSFET that handles such high voltages will have similar specs.

If you're worried about it, use one of those approaches.