I created this request just before learning about EditField("<variablename>");
with which you can draw single fields in a custom inspector. So there's no real need for that HideInAction attribute.
By using standard variables you would lose the ability to set the fields to none if you want the variable to be an FsmVariable, and I don't think you can access private variables from the inspector (at least I tried it back then, though with FsmVariables), but thanks anyway.