Hello. I am trying to place some flowers from Biomes o Plenty. But the sad thing is the BOP used their own ItemBlock logic and Block#canPlaceAt is always true. So I think this might be a way to solve this problem.

I'm confused as to what you mean. Block states are already supported via registry name + metadata specification.

I meant setting indicator state only if the block under indicator is dirt (or something)

Fixed in v2.0.5. Let me know if you need anything else.