The definition of Cremation is expanding beyond flame. Just as flame cremation expanded beyond meaning a pyre. So flame cremation is just an assumption and prejudice of the moment.
So you could just use Cremation and specify the Process in the description or as a custom attribute.
Or you can easily add a custom event. But custom events aren’t grouped in Life Events submenu nor be a “Fallback” for a broader event category. (In this case, Biocremation would be a ‘Death’ fallback.)
But if you’re deadset on having another explicit option, you can certainly hack Gramps code to add another Life Event, make it a Fallback for Death. If you want to propose it as an expansion to the core Life Events, it would also need to be marked as a translatable text string. There’s an example of this in the proposal to add “Stillbirth” as a Life Event.