PHP config files are much like PHP templates. It sounds handy to be able to do the odd quick hack in there, but on large scale project it always ends up as a gigantic mess in my experience. Quick hack over quick hack, it slowly turns into an unreadable maze of possibilities.

That being said, for packages I really don't think this applies, because it's static declarative information, I can't imagine why you would need "quick hacks" adding logic to that.