If you add a replacement for an ore and try to harvest the disguised block, it will drop or not drop based on the harvestability of the ore.

For instance:

mods.OreTiers.addReplacement("one", <minecraft:obsidian>, <minecraft:cobblestone>);

Trying to break the cobblestone with a wooden pickaxe does actually work, but no cobble is dropped.

Have you any ideas how to tackle this issue? I ran in a similar problem in my mod, where I would also need an event. What I think so far, is that ForgeHoos.canHarvestBlock would need to trigger a PlayerEvent.HarvestCheck on every code path. At least that would enable me to hook into there...

No, we have not solved the issue yet. I am going to talk with mezz and some other devs tonight and see if we can work something out.

Looks like HarvestDropsEvent is not fired when the player can't harvest the block, and forge provides no event to change the harvestable status of a block like we would need.