You said that you will add this in https://mcreator.net/comment/75506
But it didn't have a ticket ;) I know it's useless to open a ticket for this but it's just to say like "suggested"
You should add a kind of "Mini procedure" which will be a Condition for Structure Generation or even Block Generation
Like if i want to generate Stalagtites in Caves, I can make this Block generate instead of Air (not stone)
And if i want it to generate under stone, The condition procedure is here: Start Block
Generating Condition --> Get block at X Y+1 Z = Stone AND Get block at X Y-1 Z = Air