I only tried transferring to a 5J flight in Singapore, but not in HKG. You can always try if transfer security will accept your self-printed boarding pass. They often change procedures. The last time I flew with them, they will exchange your self-printed boarding pass with an airline printed one for international flights. For domestic flights, they do mark it somehow that docs have been verified, but that can be done at the gate. Anyhow, if they say you have to go to the landside counter, then it is wise to just go there. My experience in Singapore was that I was almost sent landside at the transfer desk of the full service airline I went to because they do not have "agreeement" with Cebu Pacific. They did check me in airside but "just that once". But on the way back, I have to clear immigration and go to Cebu Pacific counter. I didn't have a printer to print my own boarding pass anyway.