In case you didn't already heard from others, there's the http://xml.coverpages.org site hosting lots of pre-2000 material related to ISO 8879 (SGML) and XML. Although I didn't find too much on a quick ad-hoc search for ISO 9573, there's mention of angzarr in a preview version of ISO 9573 at http://xml.coverpages.org/ISO-PDTR-9573-13-2004.pdf by Martin Bryant and David Carlisle.
There's also casual mention of ISO 9573 on historical comp.text.sgml Usenet archives.
David and other people involved with SGML, MathML, and early entity sets for math (and chemical etc.) symbols are hanging around on the xml-dev mailing list (https://www.xml.org/xml-dev) and perhaps can tell more about the origin of that character (which looks more like a symbol for military or electrotechnical use to my totally uneducated eye).
Also, there's a typo in your post: Belisage Conference -> Balisage Conference ;)
When I tried to request it via ILL, they told me that the amount of material scanned "exceeds copyright law and scanning limits". I haven't bothered to look up whatever law that is, and I'm not sure if it's a US thing, or if it's on the UK side, and if so, whether students/faculty at Cambridge are under the same restrictions and they'd have to end up paying the same fees as well. I have a friend whose advisor works there, but I'm reluctant to ask them for the favour and potentially drag them into numerous back-and-forth emails with Cambridge Library and copyright issues...
> When you request a scan, you will be asked to confirm that you acknowledge a copyright declaration which states that you have not already been supplied with a copy of the same material, that you will use the copy only for non-commercial research or private study, nor supply the copy to another person. Copyright forms will be retained by the University Library in perpetuity. (emphasis added)
Wow! Pretty bold promise from a university that’s already been around for 1000 years or whatever. I feel like they’re really staking their credibility on indefinite document retention here lol.
My brother completed his Master's at Queen's College Cambridge and the porters there offer indefinite storage for all active students and alum. He stored his road bicycle in their storage there for something like 8 years before finally getting back out there and arranging to have it shipped. They're pretty big on retention, is my point.
There's also casual mention of ISO 9573 on historical comp.text.sgml Usenet archives.
David and other people involved with SGML, MathML, and early entity sets for math (and chemical etc.) symbols are hanging around on the xml-dev mailing list (https://www.xml.org/xml-dev) and perhaps can tell more about the origin of that character (which looks more like a symbol for military or electrotechnical use to my totally uneducated eye).
Also, there's a typo in your post: Belisage Conference -> Balisage Conference ;)
Good luck.