I’m all for animal rights, but, unfortunately, I don’t think animal shelter volunteering would count as serving the underserved. I also don’t think standard hospital volunteering (ie showing people to rooms, wheeling patients out of the hospital) would count. Hospice definitely would, and I believe it would be an interesting think to talk about.
Also, experience with the underserved is valued not only by Keck, but by every medical school. It would benefit you greatly to dedicate a significant amount of time (ie years, if you can) to serving some population of the poor / marginalized, such as those in hospice, those experiencing homelessness, or those suffering from mental health issues.