MDX - Dynamic exclude/include - reporting-services

Question

I am creating an SSRS report that will be used by a wrapper application. This application will be passing a string parameter which optionally specifies if the query should be filtered for "Regular Employees" or "Temporary Employees" (temp is a set of employee types) and I'm struggling with the best way to code this into the MDX query.
Possible string parameter values:
"Regular Employee"
"Temporary Employee"
(null)
My parameter expression that modifies the parameter value being passed into the query:
=SWITCH(
Parameters!t.Value = "Regular Employee",
"{[Employee Type].[Hierarchy].[Employee Type].&[2]
,[Employee Type].[Hierarchy].[Employee Type].&[3]}",
Parameters!t.Value = "Temporary Employee",
"[Employee Type].[Hierarchy].[Employee Type].&[1]",
1=1,
"[Employee Type].[Hierarchy].[Employee Type].&[99999]"
)
Note that I'm specifying the inverse and using an exclude so that my catch-all actually excludes nothing
And finally the MDX query:
SELECT
[Measures].[Headcount] ON COLUMNS,
Except([Employee Type].[Hierarchy].children, {StrToSet(#t)}) ON ROWS
FROM [GER]
Now, this works just fine, I just don't like the maintainability of this query.
Assumption that [99999] will never be a valid employee type key
Have hardcoded everything to exclude for the case of the single type for "Regular Employee". Breaks if data model is updated and includes another employee type in the future.
Breaks if any of the employee type keys change in the future
So I realize it's ugly, it's just the only working example I have at the minute. Assuming that I can't modify the wrapper application to change how the parameter value gets passed, can anyone see a better way to write the MDX for this scenario that is more future proof? I really wish that the keys for this data model used the string value and not incrementing numeric keys. That would be safer imo and I should be able to achieve this type of filter by referencing names .... somehow??