Hi Chris,
There isn't really a great way to handle this situation with the current versions of EF. It may be possible for the DB2 provider to invent a pseudo "bit" type in its provider manifest that doesn't really exist in the database and then let that pseudo-type be used in mappings. This would be a DB2 provider change and I'm not 100% sure it would work, but it could be worth investigating.
Beyond that, we are considering changing the way EF constrains the provider to allow mappings from columns that could result in data loss. For example, allowing a database integer type to be mapped to a .NET Boolean type. We're thinking through options here, but don't have anyone currently committed to making the change.
With regard to type conversions, this is also something we would like to implement but don't have anyone actively working on right now. The problem with type conversions is that you would likely still end up with a different model for SQL Server and DB2. This may or may not be a problem depending on your architecture. For Code First, where the model is built dynamically, it would probably not be a problem. When using an EDMX it could make things quite tricky.
Thanks,
Arthur