I've followed the guide on how to hide empty row header groups when loading data (https://10tec.com/forum/Posts/t4196-Howto--hide-or-delete-empty-group)
But want to extend so that when using SearchAsType any RowHeader groups that contain no matching rows are also hidden.
Is there an option for this already, or which event handler would this need to go in?

It's not clear what exactly you want to do. Can you explain in detail?

In any case, look at custom filtering you can do with the custom match rule in the built-in SearchAsType object. You can code any filtering logic you need with it.