On the 330, is it possible to deselect the WX RDR on the MFD? I have spotted the Co on my current Jet having done this a couple of times with CBs predicted. At night, unless you got a flash above you, it could be easy to assume nothing was painting and you were all clear. To be honest, the best tool I find for searching for cells are a set of NVGs. you can see a good 200+ miles, flashes over the horizon WELL before the typical radar will pick things up.
Would a forward looking IR camera in addition to the WX RDR be a more sensible addition? If they had a night mode on the tail cam I have seen some buses have, with a tilt feature to move from the view of the wingspan more into the direction of flight, it could be done at very little cost. Of course, doesnt help with embedded stuff, so you will still need WX RDR. Of my 2000 hours heavy jet (dont laugh), I can only think of 1 time where I needed to nav in cloud around embedded stuff; generally, if its convective, people generally stay out of clouds rather than fly though it!