Its not the brick, its the duplicator it doesnt allow custom prints.
Actually, it handles some custom prints just fine. It might be related to the fact that the Swiss Army Gun doesn't recognize the Modter bricks as Prints and so won't do the Printfill on them... The Swiss Army Gun and Duplicator might use the same method.
Someone once fixed the duplicator, but they made it private so the rest of us are stuck with an inefficient one. It sucks.