On Shadowlands it seems like the Crabe is open to any innovative way to repeal threat from beyond the wall. I would not be surprised that some prototype canon might be used against big Oni. However, since that Shadowlands forces are not just brainless armies, I guess that using it against them would mean that at some point it could fall in the hand of Shadowlands forces and might be an event bigger danger to Crab forces.
The actual setting sounds different from previous one and I would not be surprised to discover that Firearms weaponry is already in use in Rokugan, maybe in an experimental scale. However, since the Emerald Empire is a highly spiritual society I can imagine how blasphemous and unhonourable it would be to just gun down an holy shugenja.
The Idea of every one having a firearm is more of a modern concept. Especially in the US (where L5R comes from). Black powder weapon can be kept as a nobility symbol and forbidden to heimin population. FFG's Rokugan sounds more like smart pragmatic than obtuse conservative from AEG.
Anyway, as far as I am concerned L5R with guns is no longer L5R. And remember that being over the top with technologies is a choice so is to be contempt with what you already have (and Hotei would be contempt with that :3 )