I built something like this in ~2006 called "PHPinPHP" because I wanted to generate PHP classes from my mysql schema. It even used "[[[...]]]" blocks like cog does.

I eventually realized that A) generated code is completely unmaintainable; and B) the reason I thought I needed code generation is because my base language wasn't flexible enough.

Later on I switched to python and haven't yet hit a problem that I need code generation to solve.