Hello! It's true, referencing and manipulating mscorlib content is prohibited via filter manager, because any change related to this very basic dll can cause unpredicted crash, hang or anything else.
I can recommend few things here:
1. Find where is this StreamReader instance is used later in the script. If you see that the further usage of this instance is required - then you will want to find how is this instance created. For example, it's created and returned by some method, then you will include this method in the filter. If it's impossible to find the origins of this instance, then
2. You can manually change the filter file (in XML editor) to capture StreamReader object construction and configuring, but this can cause a HUGE increase of the script size with unnecessary calls to the tonns of unrelated StreamReaders used in other parts of application.
3. The best case - if you see that using this reader instance is not necessary - simply exclude the method from filter. I mean the method that accepts this StreamReader as a parameter.