How to spawn rubber on a tree (Hevea) like in ic2 mod?

Hello. I know, that is too later, but maybe I can help you. 

You should make 2 blocks: block full of hevea  juice and a empty block like this, which should have a block entity, tick updates, and a numeric NBT tag that will be updated when the block tick is updated and, upon reaching a certain value (you can configure it yourself), replace this block with a block full of juice. So, if you will click on block with juice, it will be replaced with empty hevea juice block, which will be replaced back after some time. I've given you the basic idea, it can be easily implemented through procedures.

Now about generation.

You can try to configure generation of this block like ore, which can replace only hevea logs. But I would suggest just making a lot of different geway structures with unique juice block locations, and setting up an equal chance of generating them.

About sapling... I don't know, but you should try to realize that with a random procedure.