Hmmm. Odd situation. It doesn't make sense that LA doesn't offer to even sell F/A inventory on EZE-LAX. You'd think they would still have the F/A inventory available to offer. I'm sure they're missing out on some sales. I'm sure pricing EZE-SCL in J plus SCL-LAX in F would cost a boatload extra.
I'm guessing they may give you a hard time about it, but your best bet is to call someone at LA (good luck...)