Tried thrice. Twice chat once call. Each time agent puts me on hold, goes to check, comes back saying that "as it is another airline, JL, we cannot split it." It does seem to be some sort of (pig-headed, IMO) policy.

Two of three times they said that they were going to split it then cancel, then a slight pause (uh-oh) and needing to put me on hold to check on something, and then came back with that. So, it's not like they didn't know how to split it, seemed like they did, but rather it's almost as if there is some block or notice put in the system that they use as they try to do that.

(bolding mine)

Exactly, it just feels wrong knowingly tying up scarce inventory and keep holding up a seat that I know won't be used, only to do a no show. It's bad form to do that to JL, who if cancelled would have one more seat to sell. And it's bad form to other travelers, as if cancelled it may go back into award inventory, and/or a lower fare one could go into revenue inventory. In fact, this being policy, that on CX OW award bookings pax are forced to hold flights they have no intention of taking until after flight, do a no show and then refund, could have material impact on reducing the OW award inventory available, and possibly slightly reducing the operating airline revenue also. (Hence why I said that this apparent policy is pig-headed. In fact, OW should take steps to prevent CX from doing so, as this is really inconsiderate and bad form, to put it lightly, of CX to do this to other OW airlines.)

But, alas, seems that there is no way around it...unless there is a way to get through to an agent who will just plow through whatever notice or warning comes up and do the split. Or if I can get JL to split it, then call CX back and cancel it once it's been split...but I know that CX puts a block on JL tickets (even seats cannot be changed or selected on the JL end; that is only an issue with CX, not with any other ticketing airline for JL - and according to JL, that is due to CX putting a total lock on their res of JL flights), so I don't think that JL has control of the ticket until T-24.

Technically I am sure this can be done in Amadeus. It seems to be really a policy as to why it's not being done. That also seems to rhyme with posts from others in this forum earlier who ran into the same issue (though, unlike me, in the end after multiple HUACAs they were able to find someone to do the split and cancel the ticket not needed).