No, this is not a replacement for anything.
The application and UI should be displayed on the Windows logon screen as the application helps users with physical disabilities to interact with the login screen to enter their login credentials. (note, it does _not_ enter their details for them or anything like that. It essentially assists with mouse use)
-FM
There's a complete API for that for Windows (see msdn), e.g.: the onscreen keyboard uses that API, but it depends on the input device the users are using: This is driver level, not userspace.
So if the input device is meant for the virtual keyboard and it is a mouse or mouse emulation it should work right out of the box, without adding any extra code to your application.
ALL devices that I know of that are specifically for impaired people work like that. With mouse emulation and the use of the virtual keyboard at start-up.
With the exception of visually impaired, perhaps, because I don't have much experience with that, or it is old.
I did and do a lot of these myself as volunteer.
In fpc this is not impossible, provided you have a microsoft signed binary for deployment. Costs money but sometimes you can get a certificate for a specific application for free after thorough examination by your local MS people and directly from microsoft. (little known but fact).