Never tried with EK, but with CX you can drop your bag any time same day. 11pm flight you can drop it at 7am if you want. HKIA has a central luggage facility aka CX doesn't have their own, so I think most likely you can check at Airport Express.
Would you mind checking back in how it went?
You probably already know, but today Airport Express is closed for all stations except Hong Kong Station (Central) and Airport. Airport Express In-Town check-in at Hong Kong Station, but nowhere else. Running at 10 minute intervals as normal. Protesters are attempting to congest the arteries to the airport today from 1pm, although the last time they tried they were thwarted by the authorities.
My recommendation is get to the airport by 1pm.