an update on this in case anybody in the future is interested:
since my posting the extension for piCamera input has been reworked into a new addon called : ofxOMXCamera
as part of this (perhaps because of my request or otherwise) the settings to set awgb have been exposed.
after poking around the openmax docs and this repo i finally figured out how to also expose the sensor-mode setting. if you are iterested in also using this you can try my fork of the extention here : langolierz/ofxOMXCamera
(note that although when using raspivid with piCapture you need to set md -6 , this translates to setting omxCameraSettings.sensorMode = 7; in the openframeworks extension)
i can confirm that piCapture + openframeworks works wonderfully together now – you can see my hardware video sampler project that uses it here : r_e_c_u_r