I don't think that volunteering is reasonable unless the clinic is a non-profit and you have a skill that is needed at the clinic and rare (eg. as an interpreter of a language that is uncommonly spoken in the area). If your time is valuable (and it is!!), volunteer somewhere closer to home, particularly in a non-clinical setting if you are working in a clinic (in other words, help at a soup kitchen, tutoring program for kids, friendly visitor for the elderly, etc)