At the time being there is no straight forward way of specifying this, we will be working on support for default values for the container which is a feature that will solve this issue. However for the moment there is an easy workaround you can use.
Given the fact that the fluent mapping is generated during runtime you can easily change the string value you provide to the HasColumnType based on the current backend.
We have implemented a similar solution for a customer by defining an attribute that holds the column types for different backends, that one can place over his properties and a set of extensions methods for the Fluent Mapping API that given a backend would read this attribute and set the column type correctly . If you are interested in this solution we will be more than happy to provide it.
I hope this is helpful.
the Telerik team