The source code for the current OpenSprinkler app can be found on our Github page:
The firmware source code is also there (although I don’t think you need to modify the firmware). If you want to modify the opening page to display a map instead of list, you can probably directly change the homepage layout of the app. Another way is to have your custom homepage as a separate homepage, and you can query the device for the list of stations (names and attributes) for display.
The HTTP GET commands that the firmware supports are all described in the API documents:
(2.1.6 being the latest firmware).