I wouldn't say “constitutional”, but there are a number of problems with that. I'll just give you a few.
One is the different rules in provinces and territories. They have different rules on age and mental competency requirements. Some require health card information. Some, in fact, require an actual signature. When you have 90% of people filing electronically, you're not going to get an actual signature. That's one block, in terms of the complexity and the different rules.
Also, 60% of people go to tax preparers. I think it's a real challenge in terms of having that discussion with the tax preparer—this very private discussion on whether or not you want to donate organs, which organs and under what conditions. That would be very difficult in terms of privacy. I think a lot of people would tend to just put “no” because they don't want to have that discussion. They don't want to have that kind of relationship with a tax preparer.
I'd also say that it could potentially have some consequences. People are filing only once a year, so if they put “yes” but then change their mind later on, or if they put “no” or think they've put “no”.... That decision may be irreversible at some point.