According to the AA website on system-wide upgrades:
Obtaining systemwide upgrades
When you qualify or re-qualify for AAdvantage® Executive Platinum status:
And that's it, as far as anything a typical frequent flyer can aim for in a given program year.
You get additional SWUs (previously known as eVIPs or VIPOWs) in the Million Miler program, four for every million base miles after the first million. Thus, you get four at 2,000,000 miles, then another four at 3,000,000, and so on.
SWUs can be gifted from another AAdvantage account. If you have a friend or colleague who, for example, has expiring SWUs they will not be able to use, you can ask them to have it applied to an upcoming trip of yours.
There are several things to note:
- You must have actually booked a trip, as the SWU is applied to a particular PNR.
- The SWU is not transferred to your account; it remains in the original account and is simply applied to your itinerary.
- The expiration date on the SWU is not reset.
- Bartering, selling, or trading the SWU for any other consideration is against the AAdvantage Terms and Conditions. There are all manner of gimmicks people have attempted to use to sell benefits, and the airlines have no tolerance for any of them.
As I just said, the SWU is voided if sold or traded for cash or any other consideration— but there's more. AAdvantage is a private program administered by American Airlines, and so it is the judgment of American Airlines that determines whether or not you have broken the rules. They can not only cancel the cert but take away all your other miles and status, nullify your MM progress, or close your account and ban you for life from the program altogether.
The airlines do monitor eBay, Craigslist, and other sites and investigate transactions they find suspicious. AA specifically was involved in the wholesale shutdown and revamping of the Coupon Connection at FlyerTalk over these issues, and American Airlines items remain banned there completely.
You'll find anecdotal reports on the Internet of high value customers being given a SWU as a customer service gesture in some extreme cases, like someone in paid international first class being delayed two days and then downgraded to a middle seat in economy. It is likely that such a SWU would be inscribed on dragon skin, placed in a case made of unicorn horn, and delivered by a sasquatch wearing a coat of angel feathers.
As you note, the SWU is an extremely valuable benefit, and an expensive one for AA. They have no reason to make it easy to obtain — otherwise, they would essentially be giving away their most profitable seats, and the most desirable benefit offered to their most loyal customers.