I never played MGS, but one of my best friends has cosplayed Meryl before so I'm slightly familiar with the series.
In my opinion, the people who are not too familiar with MGS may find it confusing if you, Meryl, have the box thing. Brawl just came out and the box thing is signature to Snake (one of his taunts), so I'd imagine it could possibly be confusing and not as "trademark" of Meryl to have one.
However, I don't think it would be necessarily FROWNED upon. Nothing is really frowned upon -- you would definitely get a few laughs and compliments, at least, for just having a box. I mean, cardboard boxes are awesome and random in its own right.
I offer quality button/pin commissions!!
E-mail me or PM me for more details.