Yes, UA could potentially sell an AC fare on intra-CA segments (UA does not have their own fare filings on intra-CA routes, so in addition to being CA metal, it would need to be a CA fare and CA coded flight), but it's not something you can do on website. This thread has some more backstory -- Booking partner only cash flights on United.com / app

UA currently allows for connections of up to 12 hours for Canada destinations before a stopover occurs (at which point, separate intra-CA segment would need to be on a separate AC fare instead of a single UA O&D fare from US).