The fact that the employer/employee relationship is so carefully controlled (specifically to limit the power the employer wields over the employee outside employment) is what makes me suspect that at least some jurisdictions would decline to enforce an employment contract provision that gave the employer ownership over all IP the employee produces, even if the IP is produced off-hours, without use of the employer's equipment, and not having any relationship to the employer's business.

Again, I don't actually know for sure - I'm not an expert in this area of law. But I think there is enough evidence in the rest of (U.S.) employment law jurisprudence to indicate that it's not open-and-shut in favor of the employer.