Is there something useful I can 'spend' my American Airlines 500-mile upgrades on, except domestic travel?

Based on what you have said, in the questions and comments, I think you've joined the wrong Frequent Flyer program!

As a general rule, airline frequent flyer programs fall into one of three categories. One is US based alliance, one is international alliance or large independent, and the final is budget. We'll ignore the last category for this answer (they tend to be simple $$$ spent -> $ off).

For the US alliance carriers (Oneworld, Star Alliance and SkyTeam), the main perks of status are around upgrades, and especially domestic upgrades. The higher your status, the more guaranteed upgrades you get, and the more chance you have of an upgrade generally. Most of these upgrades are US-domestic. This tends to fit with the wants and travel patterns of most US frequent fliers, who are generally US based, flying mostly domestically, and only the odd international flight. While you will get alliance status too, some of the perks (especially lounges) don't apply for US-domestic flights.

For the non-US alliance carriers, and full-service non-aligned (independent) carriers, the main perks tend to be lounge access, and extra miles. US-style upgrades (and standby!) largely don't exist. You typically look at using miles to upgrade, as well as booking reward flights, and often with more status you earn miles faster, which makes this possible. Generally, non-US frequent fliers do a lot of international travel, and very little domestic (wherever "domestic" is for home). Often, you need to get to the airport early (international flight), and if your home airport isn't the airline home you may have to change. Because of this, most of these carriers have tended to make lounge access one of their key perks.

With all of these programs, there are multiple levels. The basic level might get you a few more miles, might get you some priority at checkin / boarding / seat selection / waitlists etc, but not much. There tends to be a level that comes with business class lounge access (unless you're a US FF on a domestic ticket), business checkin, priority boarding, extra baggage etc. There tends to be a top level that has first class lounge access (US FF exceptions again), first class priority checkin / boarding, extra baggage, seats etc. There may be intermediate tiers between these.

For your flying, you're a member of a US FF program, but not doing many multi-class US-domestic flights, so you're missing out on the bulk of the perks. I'd suggest you change! If you switched from an AA program to the BA one (since you're UK based), you can still earn points from your AA flights. You would need to take the odd BA flight (you can't only fly AA), but that shouldn't be an issue - BA fly to more of the states from London than AA do! Once you've hit BA silver (Oneworld Sapphire), you'll get business class lounge access on all OneWorld flights (including when you're flying on AA on a US-domestic), you'll get business class checkin, priority boarding, advanced seat selection, extra baggage on AA/BA/IB flights, and 100% more miles (avios) on BA/IB/AA flights. You can then use these miles to upgrade some of your flights (anything except cheap economy tickets can be upgraded), as well as to book new reward tickets.

Based on how you have described your flying patterns, I think this'll suit you much better. When you're waiting to connect between your TATL flight and your small domestic one, sat in the lounge using free wifi and drinking a free beer, or even more so when you're in the BA Gallerys Lounge at Heathrow drinking free champagne and enjoying some free food, you'll hopefully agree!

Oh, and one final thing - when going for BA status, look out for cheap AA east coast - west coast flights in either discounted F, or instant upgrade F. They can be pretty cheap, and they're a great way to earn BA status very fast!

While there is a wide range of options for redeeming points at https://www.aa.com/i18n/AAdvantage/redeemMiles/main.jsp there doesn't seem to be any facility for transferring Upgrade packages to any other reward.

It doesn't look like they are transferable to other named individuals either, so you may be out of luck here...