Thanks for sharing your journey! I am trying to do the same thing
with a little fiddling and logcat I now have a working command 
-strangely enough you need to keep the %2F to get it working
adb shell am start -a android.intent.action.VIEW -d content://android.media.tv/passthrough/com.mediatek.tvinput%2F.hdmi.HDMIInputService%2FHW5 -n org.droidtv.playtv/.PlayTvActivity -f 0x10000000