What if there were a tick box on "Your Account" page that said "Donate 10L* from stipend every week for green spaces" - and the green spaces benefitting were in an umbrella organisation (like Higbee's parks). I'd do it. I believe setting it up to be donated weekly would be the way to go - if you change your mind untick the box. A lot of small amounts could add up to big bucks.
Maybe L donations could be translated into tier - bypassing the Linden exchange entirely. <--well maybe not a good idea
*or user-stipulated amount