[serxx]

I did try that and put

./tvhpiproxy.py

"./" means in the current directory. I believe your script is somewhere else. You must point it to the right place, something like this /storage/<some directory>/tvhpiproxy.py

and add & to the end, like this:
/storage/<some directory>/tvhpiproxy.py &

"&" means leave it running in the background and do not wait for it to end.

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Agreed, the fix should go in omxplayer.

Or in the Tvheadend. Tvheadend could add some code, so it always begins output with I-frame. That might be even better idea, as it might solve some problems on other platforms too. There is no point to output anything before I-frame as it's not possible to display anything before the first I-frame.