Actually, I did this with EK, booked a return, because there was a small chance, I might need a return and it was just marginally more than a one-way fare. Outbound flown and 2 days before the inbound, I emailed them to cancel the return, and requested a refund for the taxes on the non-refundable ticket. No protests and got back the taxes within a week, without having to contact them any further. No show fee would not be applicable, because I canceled the ticket and because I did not fly, I would be entitled for a refund of (some) taxes. Bottom line, the refund on taxes was higher than the difference in fare one-way vs. return.

I -think- CX would do the same.