"I see that there are two Ocean 2A/2C SIDS at HKG."
Jepp 10-3H - 3 Jun 11
"One is P-RNAV and one is not"
They appear to BOTH be RNAV SID's, specifically P-RNAV or RNP1 depending on which side of the world you come from. The only difference at first glance I see is that one is for 07L and one is for 07R
"Our filed routing will likely just say OCEAN 2A although the flight plan does include our navigation capability."
HKG ATC do not like you submitting a FLT PLN with the SID on it, they want you to enter the transition point only as the SID will be issued by delivery and may not match what you files - Can't give you the AIP reference but from a few nasty-grams that is what experience tells me
"If I want to use the OCEAN PRNAV SID, I just want to confirm that we have to specifically request a radius to fix SID from Clearance Delivery."
The SIDs you mentioned are not RTF SIDs. They are P-RNAV SIDs. An example of an RTF SID is ATENA 1E. Using the ATENA as an example, how do you get it? Ask for it would be my best suggestion. I am not sure what presentation ATC get the new ICAO Item 10 letters outlining your capabilities but I can't imagine they look at them for each A/C before giving a clearance (opinion only)
In HKG the RANDOM DEP 1/2 A/B/C/D are not RTF and the E/F ones are.
All of the above is jf the top of my head having looked at the Ocean chart only so all care, no reliability.