Could you use something other than an OpenBSD screenreader for the installer?
If you have a Braille terminal, you can install OpenBSD from a serial console. If you don't have a Braille terminal but you have two computers, I suggest using the screenreader from your present computer to set up OpenBSD and a screenreader on a different computer. Here are three alternative ways of installing the base system with a second computer instead of a Braille terminal. 1. Install OpenBSD from a serial console connected to your other computer. I think this is the easiest. 2. Use autoinstall(8): Set up network boot, and write the autoinstall script. You still need someone else to run the installer, but that person only has to press "A" at the right time and tell you when it is done; or you could even do that part yourself, but then you won't know if there was an error. Once you install the base system, you can connect by SSH or serial to the newly installed operating system to install the screenreader. If you don't want to do that, consider the next option. 3. Use a live derivative (Resflash or Fuguita), and add the screenreader to the image before you boot it. But option three is harder if you don't already have an OpenBSD system, and I don't think it will work with M1. Does any of that make sense?
