Try Splash Buddy.

You can try Jamf Helper, I used it when I was pushing the upgrade to Sierra. I had the upgrade setup on Self Service so that the user's could upgrade at their own leisure, once the preparation of the installation started I had a screen pop up pretty much stating that their machines were being prepared to upgrade to Sierra and this would take 5-10 mins and then the machine will restart. I put the -help results for Jamf Helper, this is probably what you would need to accomplish your goal.

jamfHelper -help

Usage: jamfHelper -windowType [-windowPostion] [-title] [-heading] [-description] [-icon] [-button1] [-button2] [-defaultButton] [-cancelButton] [-showDelayOptions] [-alignDescription] [-alignHeading] [-alignCountdown] [-timeout] [-countdown] [-iconSize] [-lockHUD] [-startLaunchd] [-fullScreenIcon] [-kill]

-windowType [hud | utility | fs]

hud: creates an Apple "Heads Up Display" style window

utility: creates an Apple "Utility" style window

fs: creates a full screen window the restricts all user input

WARNING: Remote access must be used to unlock machines in this mode

-windowPosition [ul | ll | ur | lr]

Positions window in the upper right, upper left, lower right or lower left of the user's screen

If no input is given, the window defaults to the center of the screen

-title "string"

Sets the window's title to the specified string

-heading "string"

Sets the heading of the window to the specified string

-description "string"

Sets the main contents of the window to the specified string

-icon path

Sets the windows image filed to the image located at the specified path

-button1 "string"

Creates a button with the specified label

-button2 "string"

Creates a second button with the specified label

-defaultButton [1 | 2]

Sets the default button of the window to the specified button. The Default Button will respond to "return"

-cancelButton [1 | 2]

Sets the cancel button of the window to the specified button. The Cancel Button will respond to "escape"

-showDelayOptions "int, int, int,..."

Enables the "Delay Options Mode". The window will display a dropdown with the values passed through the string

-alignDescription [right | left | center | justified | natural]

Aligns the description to the specified alignment

-alignHeading [right | left | center | justified | natural]

Aligns the heading to the specified alignment

-alignCountdown [right | left | center | justified | natural]

Aligns the countdown to the specified alignment

-timeout int

Causes the window to timeout after the specified amount of seconds

Note: The timeout will cause the default button, button 1 or button 2 to be selected (in that order)

-countdown

Displays a string notifying the user when the window will time out

-iconSize pixels

Changes the image frame to the specified pixel size

-lockHUD

Removes the ability to exit the HUD by selecting the close button

-startlaunchd

Starts the JAMF Helper as a launchd process

-kill

Kills the JAMF Helper when it has been started with launchd

-fullScreenIcon

Scales the "icon" to the full size of the window

Note: Only available in full screen mode

Return Values: The JAMF Helper will print the following return values to stdout...

0 - Button 1 was clicked

1 - The Jamf Helper was unable to launch

2 - Button 2 was clicked

3 - Process was started as a launchd task

XX1 - Button 1 was clicked with a value of XX seconds selected in the drop-down

XX2 - Button 2 was clicked with a value of XX seconds selected in the drop-down

239 - The exit button was clicked

240 - The "ProductVersion" in sw_vers did not return 10.5.X, 10.6.X or 10.7.X

243 - The window timed-out with no buttons on the screen

250 - Bad "-windowType"

254 - Cancel button was select with delay option present

255 - No "-windowType"

Thank you!