The latest documentation for Windows Phone 8.1 GDR1 reveals that Microsoft is well-aware of the complaints that “offline maps” data storage is not allowed on MicroSD card in Windows Phone devices. This really is a big issue on device like Lumia 530 with very less internal phone storage.
The documentation has been updated on 15th December and should be in sync with the latest GDR1 update 8.10.14219.341. So, seems the latest GDR1 update allows OEMs to enable storing of Maps data on MicroSD card and Microsoft even recommends doing so for devices with less than 8 GB internal storage.
Map data is used by the Maps application and the map control for third-party applications. OEMs can choose to store this data on an SD card, which provides the advantage of saving internal memory space for user data and allows the user to download more offline map data. Microsoft recommends enabling the UseExternalStorage setting on phones with less than 8 GB of user storage and has an SD card slot.
You can use UseExternalStorage whether or not you include an SD card with preloaded map data on the phone. If set to 1 (or Yes), the OS only allows the user to download offline maps when an SD card is present. If an SD card is not present, users can still view and cache maps, but they will not be able to download a region of offline maps until an SD card is inserted.
In fact, OEMs can make inserting MicroSD card compulsory for downloading offline Maps in devices with less storage.