My kids and I recently stayed at the Disneyland Hotel and we had this same question so here's what we learned from a friendly Cast Member. Yes, you may indeed use the Disneyland Monorail to enter the Park for Early Entry. A line will begin to form outside the Monorail station approximately 20 minutes before.

No, it is virtually impossible to sneak into Disneyland by using the monorail. While the monorail station is located in the Downtown Disney District (which is a free-to-enter shopping area), you are required to scan a valid theme park ticket and show a park reservation at the monorail boarding platform before you are allowed to enter the station. This checkpoint serves as a satellite "front gate" for the park. Once you scan in, you are effectively "inside" the park boundaries. The monorail then takes you directly into the Tomorrowland station within Disneyland Park. Because the security and ticketing staff at the Downtown Disney monorail station are just as rigorous as those at the main entrance turnstiles, the monorail cannot be used as a "backdoor" for unpaid entry. It is a convenient perk for Disneyland Resort hotel guests, but it requires the same paid admission as any other entrance.