Certainly possible and probably not that challenging to do. It would involve going into the code for Mycelium spread and changing the block type, which should be easy enough to find. The hard part would be making so it can spread to multiple block types, since I would be willing to bet it is coded in a way to be specific to one block, rather than a list of blocks that only contains Dirt at the moment.
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In fact, I actually edited it in my own mod to ensure that it could not spread to coarse dirt or podzol, and presumably Mojang did the same thing in 1.7, as well as added another change to fix the spreading of mycelium and grass underwater when Optifine's Clear Water is enabled, which reduces the light opacity of water (they use basically the same code):
A mod actually already exists that adds a "bacteria" block that rapidly spreads and consumes every other block. I don't remember the name of the mod, it's like the Virus Mod or Bacterium Mod or something like that.