Hi...how do you get a rustic box look from buttercream?  Do you spread the darker color around in random spots and then smooth out when icing and then use a cookie scribe tool to etch?  

W/o a  airbrush that’s what I would do.  I’d probably find something to emboss the icing rather than score it   -  there must’ve something that could be used 

I cannot visualize the design you are describing, so no help here.

another way if using crusting butter cream after using a fork or whatever for the texturing and it's crusted, dust it with cocoa -- 

but otherwise yes adding piped lines of darker colored icing before smoothing is good way -- you could dust after that too --

I think you could gently dust on cold Swiss meringue icing too -- 

Oh good idea using a dusting of cocoa:)