Hello,
lightdm will apparently be the default dm for non-gnome DE such as
XFCE, LXDE, MATE. ATM it never auto-start Orca, some code would be
needed for this. No keyboard shortcut is available either to start
it by hand. I'm thinking about using the greeter-wrapper hook for
this: the gnome-orca package would provide the following script as
/usr/bin/orca-wrapper:
#!/bin/sh
orca --replace --no-setup --disable splash-window &
exec $*
and then the user can easily use it in lightdm.conf:
greeter-wrapper=orca-wrapper