I think the official RICOH THETA mobile can take HDR photos with the SC, right?
If you want to do it on your own, you can use opencv on your mobile device. See the example below that is doing it internal to the THETA V. However, as the THETA V is running an Android OS, there are examples it looks to that show how to get it to work on a normal Android mobile phone.
Can you provide more information on what you intend your app to do? It sounds interesting.
HDR plugin to automatically create EXR file. For VFX use
This app might work:
http://www.bradherman.com/home/hdr-360-bracket-pro-for-ricoh-theta-vfx-vr-and-gamedev