No, it is not possible to upgrade with American miles on any other airline on any kind of discount ticket. It has to be a flight both operated by American and sold by American for you to be able to upgrade. There is no way to upgrade with American miles a Cathay flight that you purchased (whether you purchased it under a Cathay flight number or an American flight number). The only way to fly in first or business on Cathay using American miles is to book an outright award flight instead of purchasing it. (But, of course, award seats are not always available!)

There are upgrades with miles available in some cases on some partner airlines, but only if you purchase very expensive "full fare" tickets (nowhere near the price of "M" class). But that doesn't apply to Cathay anyway. See here:

https://www.aa.com/i18n/utility/jba-...servations.jsp