there are two ways how to launch d-i on live images, through the boot
menu and through the launcher (on the desktop).
using the boot menu, you'll have full accessibility support within d-i.
using the launcher, you only have as much accessibility support for d-i
as 'normal' gtk applications have support for on the live images.
not having any clue about accessibiltiy for visually impaired people, i
do not know what that means wrt/ orca or gtk or anything involved with
that. of course, any patches to improve/add stuff are more than welcome.
regardless of above, there's no difference if the included
debian-installer version does include the live-installer udeb (as it is
the case by default on the live images) or not.