I don't know about an API but there is a registry key which allows the wallpaper interval to change. Unfortunately, there is no "disable" option in the Personalization area anyways so that won't work but the interval can be pushed all the way to 1 day which may be a good enough period of time to consider it off for the time that madVR is being used in one go.
The key is at: HKEY_CURRENT_USER\Control Panel\Personalization\Desktop Slideshow @ Interval
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