Good day everyone,
I've managed to make some basic block modifications.
The Serial print block has been locally modified to include the text "OWIEOWIEOWIEOWIE" in Blockly and now attaches a comment "// bazinga" when translated to C code.
The noTone block now has a custom hex color of f92f2f (a specific hex color I think is my favorite).
Finally, there is now a "sandwich" block in a new "Foods" category that accepts one parameter and completely ignores it. It declares the const int SANDWICH in the C code.
Reshaping a block is not feasible without reversing the Blockly compression toolchain (i.e. working from an uncompressed copy of Blockly and recompressing it with changes to block rendering), which is something I haven't yet found time for.