There is an issue for this already. If it gets enough upvotes then we’ll get to it:
We could allow users to choose 3 custom default presets in the application settings. You can add a feature request in the issue tracker and see how much upvotes it gets.
In general, extensions should not modify user preferences (without explicitly getting approval from the user). So, probably the most appropriate level where an extension can inject its custom behavior into the application is to add a subject hierarchy plugin that recognizes certain volume types where the default VR preset is not optimal (e.g., I can imagine that you can create better preset for dry bone), claims those volumes (gives higher confidence value than all other plugins), and uses your custom volume rendering preset when display in 3D view is requested.
If the problem is that in general you don’t find the default VR presets optimal then we can change those presets or add new presets that will be used as default.