Doing a little more research on the HU and tablet control, it looks like the same Joycon guy has a module that allows you to control both simultaneously (configuring either a toggle button to control, or long press/short press to pass through commands to each endpoint).
The english is horrible but from what I can determine, its fully programmable so may be a way of solving my scenario:
http://rcjoyeng.cafe24.com/base_3/me...m_board_page=&
Video:
[ame="https://www.youtube.com/watch?v=nTgNNVd_yWQ"]Control both Android and Sony radio with one steering wheel controls - YouTube[/ame]
Should be a fun weekend project!