One daughter attended a high school that did not offer any AP classes. She self studied for one AP exam, specifically AP Spanish. Her Spanish is very good (she had already studied for three months in South America at a school where all of her classes were in Spanish – we had also already hosted an exchange student from South America). It was very difficult to find anywhere that would let her take the AP Spanish exam, but we were able to eventually find somewhere.
I do not think that this had any impact at all on her university admissions. Her university gave her credit for first year Spanish (with a 5 on the exam). This shows up on her university transcript with a grade of P (pass).
I would not do this for the purpose of helping your university applications. Also I would be very cautious about doing this for any math class since math is a subject where everything that you learn is so highly dependent upon what you were supposed to be learning last year and the year before. In math it is important to learn each step very well.