Hi,
Have you studied my XmlMaker asset? it has some pretty advanced action editor and custom UI, this should give you a good headstart.
In your case, you may simply in the end have regular Fsm variable public vars that you simply hide show in your custom editor by not displaying the one that do not fit the current context.
for example: use EditField("MyField") in your custom action editor, and simply have "MyField" be the right variable name for the current context. So let's say you have "MyString" and "MyFloat" as public fsms ( FsmString and FsmFloat), you either do EditField("MyString") OR EditField("MyFloat"), and it will only show that one.
Does that make sense?
bye,
Jean