AFAIK there's no way to do online check in. I don't think I can even see my hotel reservation on my online account when I log into the DLR website. I booked my reservations directly through the DLR website. Last yr we went to WDW and I booked directly through the WDW site--during booking you had the option of online checkin, and could log into your account to see your reservation, ADR's etc IIRC.
In the past, GCH used to have a phone number you could call perhaps 2 hours before your arrival to do a "pre check in" but I don't think they do that any more. In general though, I've not had to wait too long for check in at the front desk (though maybe it's the day of the week or time of day that determines the wait). At GCH, DH & I have a routine: he pulls up to the front, we have bell services grab the bags, I take the kids to the lobby to watch movies & I stand in line for check in while DH self parks the car.