Unaligned Exception Handler is not intended to be set dynamically at runtime.
The default unaligned exception handler can only be statically updated. This is by design so that the unaligned exception is executed "fast." It runs straight through and gets the job done (normal exception handling was sacrificed for speed).
You can set the default unaligned exception handler statically through the XPS GUI (Software Platform Settings > Library/OS Parameters, Standalone > microblaze_exception_vectors).