I would signup to MyPoints (1)(2) with a random US zip code. They will not activate your account but then you can e-mail them saying you are traveling*. I haven't tried but you can also use the Opinions Miles Club as well (3).

Mileage Plus Shopping works even if your account isn't based in US. But since it most likely involves delivering a physical good in US, that might not work for you. The MileagePlusX, where you can get a lot of online services (such as Uber gift cards and so on), as you said only works if you are physically located in US (the app uses your geolocalization) regardless of your account.

Hotels: you can also try to book a hotel at United (4) if you have any trip soon before the one Japan/Taiwan. They usually credit the points fairly quick after the checkout date. You can also use Booking.com (5) or RocketMiles (6) too. Note that points will be credit after checkout in all these cases, so this is the case if you have any upcoming trip soon where you make the points then redeem for your Japan/Taiwan trip.

I would even consider signing up to The Economist for a Digital subscription for £12 for 3 months (7) which would give you 1k points. Read the fine print to see if it would work for you, I never tried as it is not valid for current subscribers (which is my case).

(1) https://www.mypoints.com/landingPage?cmp=1261
(2) https://www.mypoints.com/landingPage?cmp=911
(3) https://www.opinionmilesclub.com/gen...09.87.115.194&
(4) https://hotels.united.com/
(5) https://shopping.mileageplus.com/me____.htm?gmid=4791
(6) https://www.rocketmiles.com/united-3k-bonus/
(7) https://shopping.mileageplus.com/me____.htm?gmid=1926

* This is actually what happened to me – it looks like they were looking my IP address, my account is non-US as I live in Europe but I got the points. My girlfriend did the same but over VPN with a UA US account, and their support said VPN might get the activation delayed as they don't support that. They activated the account manually. Very strange but it worked out for both of us.