I'm not sure but to me it just looks like that class is missing? It maybe that you have it in classpath so it works in test but not on cell? I think you just need to stick that class in the package.
When I work with bouncy I always use it at source level, ie I dont use libs. Its good but its full of excess stuff, and I pull out as much as I can once its working. I havnt used it on cell, but I think there trimming it down is even more important.
I remember seeing a few article on bcy and cells... probably worth a google.
subject: Using BouncyCastle in midlet -- Error in console