"This one looks to me like human rights challenge in the making: is it not discriminatory to forbid a right to people with a mental illness that is available to all other Canadians?"

Yes it is discriminatory, and in my opinion that aspect of the Liberal's law would be found unconstitutional if it reached the Supreme Court. 

Legal challenges have already been filed against the law's restrictive provisions, however I don't believe they've been filed on behalf of patients with mental illnesses. It's likely the Supreme Court wouldn't hear such a challenge as long as the government is honestly reviewing the matter in committee, as they've promised to do.