Unfortunately granting Brutaldon access to Mastodon was a failing prospect under retro-proxy... that's because the designer included only GET support and little else - enough to browse but not enough to click forms or comply with modern security issues (CORS, etc). To that end I forked retro-proxy and added my own patches. My version, which I'll be submitting a PR for shortly, supports more features needed to get brutaldon working:
* "Manual" redirection and auto-detection of "forced HTTPS" sites
* PUT, PATCH, DELETE, etc. methods beyond just GET
* Form submission with BODY payloads
* Cookie support
* A hack for OAuth to let me get my initial post made :P

You can see my version here:
github.com/greg-kennedy/retro-
but, with any luck, my work will get merged back into the main repository and everyone can benefit :)

Aaaand... that's it! Virtual DOS on DOSBox, running Windows 3.11, with Trumpet Winsock making a SLIP dialup call to SLIPGateway, allowing IE5 (through a compatibility shim proxy) to make a Fedi post in the wrong hashtag :P

screenshot of my Mastodon account login from Internet Explorer, which is totally IE5 I SWEAR
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If you have a fediverse account, you can quote this note from your own instance. Search https://icosahedron.website/users/greg/statuses/114168199429847368 on your instance and quote it. (Note that quoting is not supported in Mastodon.)