[1.7.x] Lava cobblestone and stone replacement

Hi,

I'm trying to figure out a good way to change the block that is created when water hits non-source lava blocks. Currently cobblestone and stone (when lava hits water) is created but I want one of my custom blocks to be created instead (Pumice).

I have tried a few things:

• Replaced cobblestone with pumice using the new Substitution functionality in latest 1.7.10 dev releases. This caused NPEs because of what looks like a bug with Substituted blocks not getting the texture correctly registered and it also replaces all cobblestone in the game.
  
• Used the same function as above (Substituted blocks) creating a block that spawns a TileEntity which in it's update method replaces the block with my block and then removes the TileEntity..    this works pretty well (
  ) but eventually causes random freezing of the game.  I guess it's because it creates too many TileEntities when a new chunk is generated where there are lava and water underground. This functionality doesn't really work good either way since it will only work on Cobblestone..  Doing this on Stone (which is the core block during Worldgen) is insanely slow.
  
• Not tested an implementation but looked into possibility to extend the Static and DynamicLiquidBlocks and override the function that sets the stone or cobblestone and use this as a substituted block but run into issues since the method is private. (might be able to solve it with AccessConfigs) but will probably cause same issues with rendering as I had in my first attempt.
  

Anyone that have any brighter ideas than this how to solve it?  Dream scenario would be support in Forge for it so the block types created could be settable and not hard coded to Blocks.stone and Blocks.cobblestone.