If you are considering sneaking in anything to affect the piss that is being tested, why not just save any and all hassle and sneak in some Quick Fix?

That way you dont have to worry about putting too much or too little, or it smelling funny or anything else.

At least that is what I would do if I was planning on sneaking stuff in to alter the test. If you are already having to hide things, why not hide somethign that is easy and promised to work.