Just an update, I can confirm that I was successful sending legal documents from USA to Russia via Registered Mail utilizing the United Nations Headquarters in Austria for mail relay, due to the continued suspension of all mail delivery services to Russia by USPS.
The process involved first buying Euro-denominated UN-postage stamps in USA from the UN Postal Administration (UNPA). In my case, I actually went into the UN Headquarters in New York to buy the stamps at their postal store in the visitor’s center (see photo). Yes, it is really possibly to get a visitor’s pass to enter the complex without a tour scheduled, just for this purpose! They opened late (was supposed to open at 9AM), but they did help me.
To prepare the letter for mailing from Austria, I weighed it and priced it out using the Austria Post website, including the extra fee for registered service, and added enough UN stamps totaling that amount onto a #9 letter envelope (see photo) to pay for it. UN stamps are big! But also the usual #10 envelopes are oversized by European standards. I also included with the stamped envelope a cover letter in German and English addressed to UNPA itself describing that I wanted the letter sent registered and included my Email address, plus a print-out of the website price calculation and service restrictions (only documents may be sent to Russia from Austria), and further a UN-stamped, self-addressed envelope for UNPA to use to provide the registered mail receipt and tracking number. Then I mailed all of this in an outer envelope by USPS First Class International Mail to the UNPA address in Austria.
About 2 weeks later UNPA received the letter and emailed me that as Austria Post does not allow sending registered mail using postage stamps, so they asked permission to send the documents to Russia in their own envelope instead, which I agreed. A few days later they emailed me the tracking number and registered mail receipt. They never used the stamped, self-addressed envelope, so I guess they kept that additional postage as an informal handling fee for doing all of this!
Tracking of the letter reveals it took about 2 weeks to get transferred to Moscow. More tracking details are available using Russia’s postal service website than Austria’s, though you need to use a VPN to access them for some reason. Finally, about a week later, the registered letter arrived at its destination and a notice was left with recipient to go to the local post office and sign for the article. The final envelope received (see photo) had my original addressing label transferred onto it.
The strangest part of all of this is that it actually cost me less to send a registered letter from USA to Russia via UNPA in Austria than it would cost to send it directly from USA if that were possible, because USPS wants over $20 additional now to send international registered mail, but Austria Post only charges about 4 euro additional.
Mission accomplished!