Would there be any way to change the hardness value of a meta block, like diorite?

Override getBlockHardness and make it depend on the meta I guess

If you want to manipulate vanilla blocks you have two ways - replace vanilla and write whatever you want, or use Events.

 

To change hardness of block you can use PlayerEvent.BreakSpeed, but that will be only for players, not e.g explosions.

Aah I missunderstood The question ernio is right

I had a reason to replace obsidian with a custom version in order for it to work properly in a dimension mod I have.  My custom obsidian had the exact same properties as original obsidian but it enabled me to have it respond differently for portal creation. 

 

I watched events to see when a block was placed or interacted with in any way.  Then if the block was vanilla obsidian, i would change it to my custom version as well as do a small iterative search that replicated itself for each obsidian in the area found.

 

This worked very well, but the world isn't very populated with obsidian.  In your case if the block you are changing is very common, this might get ugly the first time it propagates.