Anyone has tried to plant fig trees in a medium based on rice husk ash? Any experience? I worry it is too alkaline...

I wouldn't plant anything in any medium based on any type of ash.  It will be very alkaline.  You could perhaps mix it with something that's acidic.  But ashes have no structure so not that great for a medium.  OK as an amendment in some circumstances.

Hi,
Too much of ashes will "burn" a plant whatever, the medium, you're adding them to. It is the same with giving too much fertilizer at ounce.
So in pure ashes, I fear that you have no chances of getting a tree to grow.
If you have a guinea pig, try it. Don't chance it with a valued tree ...

By the way, could you shorten your signature to make your posts shorter ...