The principle question to ask oneself is...'by volunteering am I taking an Indonesians job?'
I know some foreigners on retirement kitas/kitap who volunteer for organisations like Rotary, Animal Welfare (BAWA), Smile, Soleman and no problem from Immigration.
Here's a list of charities in Bali...I'd suggest you call any selected one and find out the legal requirements.
http://www.balispirit.com/community/ngos
A multi-entry Sosbud is available...It lasts a year but it requires one to exit Indonesia every 60 days and return. I had one but found planning a return trip to S'pore or KL too onerous. I even tried to pass immigration in Jakarta, avoid the flight, and return to the arrival desk but was caught and scolded. I had to take the flight to KL, buy a duty-free vodka and return on the next flight...such a stupid waste of time and effort never mind the cost.