Here are draft specs. for accessibility in next version of Slint's
installer. It includes a suggestion from Philippe to always provide
speech when starting the installer. 

When the boot loader's main screen is displayed the user
_ can choose a language with the up an down arrow keys
_ can append " s" to the command line to say that she wants speech
_ can append " brltty=<driver code>,<device>,<text table>" to the
  command line

Both brltty and espeakup daemons are always started, regardless of the
command line's content.

Once the system has started, if the word s|S is not found in the command
line the user is first asked in English to type s to tell that she wants
speech, else just press [Enter]

Then, the user is asked to confirm the language set in the command line
or choose another one among those available.

If a non Latin 1 language is chosen and speech is set, the message will
warn that speech won't work and ask to re-confirm the language or choose
another one.

If brltty is set, the setting is written in /etc/brltty.conf using bp2cf
so this setting will be the default in the installed system.

If brltty is not set, but a Braille terminal is connected through USB it
should work (using udev rules), possibly with a wrong text table (that's
what Philippe remembers using brltty 5.4. Samuel, do you confirm that is
still the case?)

Rationale:
_ A user who have read the instructions can pre-configure the installer
  from the boot manager's menu.
_ A user who didn't can still use the speech synth and/or a Braille
  terminal during installation.

I don't know if a Braille terminal connected through a serial line works
during installation even if brltty is set, as AFAIK this has never been
tested.

Comments, suggestions for enhancements are warmly welcome.

Have a good day,

Didier