Is there any way I can get a dual-band cdma phone (e.g. sanyo RL7300)
to work in Hong Kong?

It is way too complicated to bring a CDMA phone to use in overseas.
Oversea carrier will need to reprogram the phone with their settings
in order for your phone to get on their network. And in most cases
oversea carrier will not support that particular phone you bring over.

I think Orange is the only carrier provide CDMA service in HONGKONG,
not sure if they still providing the service or not. Most easy way for
you is get a cheap cheap phone when you get there and use a pre-paid
sim.