Does pro bono always mean taking on an entire case?

No. There is a broad range of pro bono activities that do not require direct, full representation and court appearances. Many programs run clinics and hotlines. Your only commitment is for the time spent at the clinic or covering the hotline. California courts allow unbundled (or limited, discrete) services, so that you can draft pleadings only or take on just a portion of the representation without committing to full representation.