The rules actually say that it should be possible, but Alaska's online system doesn't let you do it.

If you search a simple RT with KE it will give you the discounted pricing - however, any sort of multicity/OJ will error out on the engine, even if both destinations have availability on KE.

You can HUCA into AS and ask them to do it as their terms do say it's possible. Another thing you could do is ask them to book it and refund you the excess of miles if you get an agent to agree with your terms.

It looks like the agent is not allowing open jaws, but will allow you to terminate in HKG - this is probably what their system allows, since it's an actual round-trip. I would probably escalate but have plans to book a separate HKG-KUL if you needed to take this space.

edit:
I'm actually quite disappointed by the responses. "What is the advantage of booking as a return? Why do you want to travel on Korean?" and "I've never heard of such a thing", but no one has actually tried it themselves? Here's a screenshot of two different partners, one as KE, with half-RT pricing!