I agree with Markit that 'volunteering' is a very grey area.
If you get here and living in the sticks, as you describe your future, I'd suggest getting to know the banjar first and offer yourself...if they agree, you are home and free, as they control village life.
Just don't go and upset anyone outside the village because 'using immigration' is how Indonesia retaliates to foreigners they don't like.
For example... here is current news of a Fullbright scholar who attended the tear-down of those riverside huts this week...Jakarta Post...
"The South Jakarta Immigration Office arrested Fulbright researcher Frank Sedlar during the eviction in Bukit Duri, Tebet, South Jakarta on Wednesday for an alleged visa violation.
Sedlar, a civil engineer from the University of Michigan, the US, reportedly aroused officials’ suspicion as he had come to the eviction site with advanced photography equipment, such as tripods and a high-tech camera, like a professional journalist....."
His accompanying friend, on a VOA, was being deported.
It's clear they don't like interfering foreign activists.