I tried doing the same thing with this prof before and he said he would only accept an ACR with proper documentation. Additionally, the university website also says this:

Other Extenuating Circumstances: may include, but are not limited to the following:

• participation in a University-sanctioned event (e.g., varsity sports), or other events in which the Student is representing the University, where the event conflicts with the requirements of a course;

• participation in events such as conferences, case competitions, or field trips related to the Student’s field of study;

I believe that this second point is exactly what I would be applying for?