This is how I'd do it:
we see him with his mask on AND we see him with his mask off.
...but we don't know it is the same person.
Sure, fans will know that "Mr. McCullen" is Destro, but the general audience wouldn't.
It's similar to the way they approached Ra's Al Ghul in Batman Begins.
In a way, it's also like a Superhero/secret identity thingy. As McCullen, he hits the "jet set" scene and could even be regarded as a humanitarian who will be awarded a Nobel prize sometime (?), but in reality, he is the terrorist/arms dealer codenamed "Destro", who the Joes are after. And he's connected to this new "sect"/"faction" of sorts named "Cobra".
So... we see both of them but we only get to know that they are the same person towards the end... when McCullen would be forced to abandon his public life forever.
See... this could work perfectly for scenes like this:
You could have McCullen at some jet-set event with politicians, humanitarians and such... and the Baroness would show up, presenting herself with some fake ID. And while the general public would think that the Baroness is trying to get something out of McCullen, the fans would know it's actually all an "act". HELL, McCullen could even be kidnapped or have his life threatened... and it would all be part of the big scheme, whatever that may be.
This... I command!
EDIT: BTW,
"Masks Off"
EDIT 2: As for the "look" - I'd have his mask be "one piece" (no movement of any sort) and, at least for the first film, it should be silver.
We could also show the original ancestor mask. Maybe it is safely kept in the "Destrocave" at McCullen's castle.