Loopholes
Lazer: I'm intrigued (or maybe I'm just bored silly at work) so I'll attempt to map this out, step by step if someone were to travel (by plane) to Canada and from there to their home country. I see some problems however:
Scenario #1: staying inside airport
1. Asylee travel from US to Canada. RTD does not get stamped. Asylee doesn't have to fill out the arrival/departure card for Canada (which they do, if I remember; you keep one half of it, which has to be returned when you leave; note that on the card itself you have to fill out the number of your passport/travel document).
2. Asylee boards another flight from Canada to country of persecution, using his/her national passport.
3. Asylee returns from country of persecution to Canada. Note that RTD or passport is still not stamped by Canadian borders (because you're still not leaving the airport).
4. Asylee boards another flight from Canada to USA.
5. At the USA border, officer flips through your RTD, asks you if you're returning from Canada but how come nothing is stamped on your RTD to prove you enter/exit Canada?
Scnenario #2: coming out of airport and reentering
1. Asylee travels from USA to Canada. Goes through Canadian borders with RTD. Fills out Canadian Arrival/Departure card, noting your RTD number. RTD gets stamped by the Canadians. You keep one half of the Arrival/Departure card.
2. Leave airport. Then come back to board your flight to country of persecution with your national passport. Canadians will ask for your other half of Arrival/Departure card, but then, it won't match up with your RTD for the document number you filled in right? Potential problem here.
3. Say if asylee gets through somehow, flies to country of persecution with national passport. Comes back to Canada. Have to fill out another arrival/departure card, indicating passport's number. When you're ready to board your flight from Canada to the USA with your RTD and turn in your arrival/departure card, numbers still aren't matching right? Another problem.
My feeling is that what messes you up is that most countries require you to fill out arrival/departure cards, which they then use to enter your passport/travel documents numbers into their computer system. I don't know how you can plan the whole thing in a way where on your RTD, it clearly shows the INS officer that you spent say, 2 weeks in Canada (with proper entry and exit date and time stamps) when you return to the U.S.