To make the game run on HOR+, you will need to either change the FOV line in the attribute in the mbpPrefs.cs file located in the client directory of the game:
AppData\Roaming\Marble Blast Platinum\platinum\client
Replace the values in this line:
$pref::Player::defaultFov = "FOVnumber";
Regular:
1. 90 (game default, pan and scan)
2. 100 (16:10)
3. 116 (widescreen)
Open Matte:
1. 116 (pan and scan)
2. 139 (16:10)
3. 142 (widescreen)
Warnings: if you are using a value higher than the maximum value intended for the game, do not enter the options menu otherwise it will set it to 140. The game may not have very good panoramic widescreen as the FOV has a fish eye effect.
Update: Download the exe in the files available from the PC Gaming Wiki Community Files (search there)., extract the contents and replace "marbleblast.exe" in AppData\Roaming\Marble Blast Platinum with this file and launch the program to see if it runs in widescreen while maintaining the top and bottom.