A very significant portion of spells in WoW are instant cast yet AFAIK neither oUF nor the default UI provides a proper means to track them, hence players have to rely on inferior means such as sound / game world effects / warnings specifically set up for spells of interest.

There are no fundamental differences between instant and cast time spells from an interface or gameplay perspective, not even their "cast time" as more often than not that equals the global cooldown. They are mutually exclusive hence castbars that are readily available with the exact features required to display spells of any kind sit idle during the global cooldown triggered by isntant cast spells. I have always considered this a shortcoming of the default UI; why make people fly blind half the time when the solution is already there?

Is there a possibility to add this to oUF core or alternatively ask for minor modifications to the castbar element to accomodate this as a separate module? I have implemented the feature for personal use by modifying castbar.lua if you are interested, although at present the code is sketchy at best.

The general rule is that the implemented scope of the element is decided by what Blizzard provides. I can say yes/no to modifications that help you implement it externally, but I don't think this is anything that would be implemented in the core of oUF.