I know I'm a bit late but I don't see any other answers on the web.
I've just managed to achieve what you wanted and you were on the right track. The only thing you may have missed out was that you need your tvheadend username and password to be able to download the tv channel stream.
First obtain a playlist from tvheadend.
wget http://<tvheadend IP addr>:9981/playlist --http-user=<tvheadend username> --http-password=<tvheadend password>
The downloaded file will contain http addresses for accessing each channel.
Now create a FIFO
Now use wget to place data into FIFO
wget '<http address for chosen channel>' --http-user=<tvheadend username> --http-password=<tvheadend password> -O /tmp/tvpipe &
Make sure to put the & on the end so that it runs in the background. Also the channels http address should be surrounded by the single quote marks.
Now use omxplayer to play the FIFO data