Ok i just ran a quick test with this module:
https://gist.github.com/FireFart/9517509

Output:
```
Should be:
32%29%20UNION%20ALL%20SELECT%20CONCAT%280x7163756871%2CIFNULL%28CAST%28HEX%28LOAD_FILE%280x2f6574632f706173737764%29%29%20AS%20CHAR%29%2C0x20%29%2C0x716f687671%29%23
After MSF:
32%29%20UNION%20ALL%20SELECT%20CONCAT%280x7163756871%2cIFNULL%28CAST%28HEX%28LOAD%5fFILE%280x2f6574632f706173737764%29%29%20AS%20CHAR%29%2c0x20%29%2c0x716f687671%29%23
```

As i can see so far:
_ is replaced by %5f