I believe there is already a way for travellers to register with Global Affairs Canada. However, while there may be some motivation for people to register that they are travelling somewhere, they have, I suppose, less motivation to say they've come back. Sometimes that type of thing can mean that the department ends up with a list of people they think are travelling who are in fact back in the country.
I believe it's very difficult to get a precise list of who needs to be contacted. The better the information they can get, then the better contact they can make, but I think that no matter what information they collect or how they collect it, they also have to have ways to contact Canadians more generally, because they're never going to have a complete list.