So I went to do a Norbert's Gambit trade on an interlisted stock at RBC Direct Investing and got the following scary error message.
Turns out, after I call customer support, that this message is just a warning. As long as you properly execute your trade you will not receive any currency conversion fees.
One side effect of this is there was a 20 minute delay between my buying and selling stock. I've always managed to do it in seconds. The end result was a 0.0754% effective conversion fee on the trade which is fine. The morale to the story I guess is, while separating the buy and sell increases risk, generally speaking it's not the end of the world if you have to call in to confirm details before finalizing a trade.