Actually, you are right and I'm mostly wrong. The Maastricht Treaty just says:

Any European State which respects the principles set out in Article 6(1) may apply to become a member of the Union. It shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the assent of the European Parliament, which shall act by an absolute majority of its component members.

So, what is a "European State" is effectively just a political decision by the Council and Parliament. I guess if Cyprus and Armenia were considered "European States" then Canada is not that big of a stretch.

Additionally, the next paragraph is

The conditions of admission and the adjustments to the Treaties on which the Union is founded, which such admission entails, shall be the subject of an agreement between the Member States and the applicant State. This agreement shall be submitted for ratification by all the contracting States in accordance with their respective constitutional requirements.

Soo I guess even the Maastricht treaty allows itself to be modified, maybe removing the "European" criteria completely.

So it's not as difficult as I first imagined, it's just a question of political will from Canada and the EU institutions.

It has been tested and Morocco didn't make the cut. I think it'd be a tough sell to argue Canada does when Morocco doesn't.

Canada technically administers a small parcel of land in France, and Canada has a land border with an EU state.

I do think that if Canada was genuinely prepared and unambiguously politically willing to join the EU, that the rules would get rewritten. Canada would be the 4th largest economy in the EU.

I think any hesitation on the EU side would be basic trust that they aren't going to get jerked around by another primarily English speaking country, or have the country fall prey to unsavoury North American politics. I feel like Canada would need to do some PR work to distance ourselves from the UK and USA.