intrusive transparent non clickable top layer?

[Sylvain]

Hi,

I would like to have a very visually intrusive count down on my screen
at scheduled time.

The count down is grayed and transparent, it doesn't change the behavior
of anything on the screen, it takes all the screen.

A grayed layer, visually intrusive, but non blocking, would be
advertising that an event will come in 10 or 30 seconds. A kind of
transparent, non clickable screensaver…

Do you know such application?
Is they're a way to program such application, where should I start
looking for?

Regards,
Sylvain.