AA only offers free MCE seats to selected oneworld elites. I can only remember AY BA JL QR are those selected partner carriers (maybe more) but I'm sure CX isn't one. So even CXDM won't be able to choose MCE seats for free.

However if your AA booking has no seats selected, their system usually automatically assigns you an orange MCE seat at checkin (both online and at airport). So I often call up to AA to get my pre-assigned green / blue seats removed to maximise the chance of scoring an orange seat