Does the sign not have its own OS, and program to support video playback installed, the idea would be to load the video onto the sign, and use the signs onboard player to play it at start up.
I have never done this, but given what i read on the Raspberry Pi forum, a small microprocessor such as a Raspberry Pi is whats installed in such devices / kiosks / digital signage.
If you know the IP type ssh root@the ip address and enter your password when prompted, In the example above I used the user root,
but given you know the password, then I would assume you know the user for this password, so again, sss whateveruser@ipaddress and enter your password, if you get logged on, type uname -a that will show the OS that is installed, and give you a starting point.
Hopefully others who know more will come along and help.