To get more detailed, and controllable information (such as being able to select a GPS device), a developer has to use code libraries from else where or write the code himself to communicate to the GPS (internal or external) devices.
OK; I see that. But it's not hard because every normal GPS does bluetooth and NMEA is standard. That is how the "GPS world" worked before Jobs did it his own way....... and compatibility was IME excellent across products.
What I don't know is whether it will ever be possible to use any normal bluetooth / NMEA GPS with the Ipad, because under Settings none of them show up at all. Jobs seems to have banned the support of a normal "serial" bluetooth device. But evidently units like the Dual do somehow deliver a full-featured data stream over bluetooth, into which they can stick any data they want, and presumably this could be straight NMEA again (anything else would be totally illogical in marketing terms).
I do hope that Oziexplorer does this correctly and not just uses the stupid IOS GPS API...