Have you tried ObsidianBreaker? https://dev.bukkit.org/projects/obsidianbreaker Allows you to add select blocks and the number of blasts they take...
You should use ranks for that as OP has all permissions. Had a go at this plugin last night however you didn't reply to my pm. Link:...
Isn't this already a thing within mcMMO? The wiki suggests so.
Separate names with a comma.