When developing a provider it's possible to create provider specific configuration keys that administrators can use to change parameters for your provider. These must be defined at setup time in the machine.config registration of the provider otherwise any attempt by the application to create a ConfigItem object and pass it to the RBS client library will fail. This can be specified as follows:
<Provider name="file"> ... <ProviderSpecificConfigKey name="pool_capacity" format="Number"/></Provider>
Formats allowed are Number, Name, Boolean, Binary and Duration. When registering the blob store with the RBS instance, the stored procedure mssqlrbs.rbs_sp_add_blob_store is used. This takes an XML configuration object, where specific values for these custom configuration options can be specified.
Correct registration of the provider and installation using the administrative stored procedures can be done using the InstallProvider.exe tool (full source and binaries available) from http://www.codeplex.com/sqlrbs.