Thanks for the response. Unfortunately that doesn't answer the question. I'm not having trouble enabling the assistive technologies and the keyboards (Caribou, to which I refer in my original post, is the defualt keyboard to with you refer in your post). The trouble is after they're enabled.
The issue is that by default in GNOME3 the shell will "grab" the screen when a password dialog box pops up as a security measure (and also when the run dialog comes up, as it happens) and as a by-product will not allow use of either Caribou or OnBoard to enter passwords. While the onscreen keyboards are present, touching the virtual keys does nothing to enter text in the dialog box. When the box is remove the onscreen keyboards return to functioning as normal.
There used to be an option within assistive technologies to treat password dialog boxes as regular windows, it was a check box, which kept GONOME from allong with windows to grab focus. This would allow a user to accomplish text entry with the on-screen keyboard of their choice. In GNOME 3 this seems to have disappeared as a check box, and I have not found a matching option in so I am looking for information as to where the ability has been moved.