Not an answer sorry, but just to comment that this could be really quite useful!
Alternatively an 'Info' tab, or a button in the main Decent Sampler top bar (or in the 'File' drop-down even) which would bring up any information the author wanted to include for the loaded instrument. That way it would be easily accessible without cluttering your main UI.
Another idea might be if the new drop-down menu could be augmented to allow a long text field instead of selectable options.
Also I guess this text could either be included in the .dspreset file itself or fetched from a 'readme' file.
Anyway, rambling sorry, but good idea!