"Both a Mirror and a Hammer."

I took it to be a metaphor of the opposites present in WoY's music - dark/light, heavy/soft etc. Similar to your lyrical idea really :)
 
You basically got it!

Visual art is sometimes seen as either a 'Mirror' (an expression that is a reflection of the self or society) or a "Hammer" (art used as a weapon against others in society and sometimes even against the self). Woods of Ypres is a reflection of the self and it's place in society, used as a weapon against that society. "Both a mirror and a hammer." :)