We have had a workaround solution since moving to V11. This was not a problem in System 9.
The issue with blanks not defaulting to [None] is that some values escape the test. Such as forcing default values using "Like (* = [None])". If a new code is in your data that is not in your map, the Validation process foiled since [None] is valid and all blanks are now [None]. Only then do you either have an intersection violation that shows up during Export or you get bad data in HFM.
I don't like this new "feature" in S11 FDM. We will be submitting a request to have the default functionality added back. Perhaps as an option?
Just for your awareness 892775, we had used a similar approach when we did an upgrade of FDM, but we found that the conditional map scripts had a significant impact in the performance using this. We went down the path of putting a datapump script on the import format to make null fields actually return a value of "Blank" and then used explicit mapping to map Blank to [none] and it had a dramatic impact on our performance.
Just my 2 cents