So it happened a few times to me that i render a song to disk, and there’s several seconds of silence at the end of the resulting .wav file - seems the disk rendering process IGNORES the ZT00 commands and goes on towards the end of the last pattern …or the entire orderlist (haven’t tested) - but this is definitely a nuisance, at least in this case –
http://soundcloud.com/delt01/we-shine-so-bright
– where i noticed this only AFTER uploading the tune to soundcloud 
Solution: Listen to the song or open it in some program before uploading 
But really, I always listen to the result of a render to make sure it came out okay, as well as open it in Goldwave to save the mp3 version. I would never just blindly upload a song after render.
Yeah agreed 100% (above was my stupid mistake) …but come on, if ZT00 markers worked as they should, it would be kind of less annoying 
for ex. the tune above, the ONLY edit i would have had to do on the .wav file, was cut the silence at the end.
The same thing has happened to me…the rendering process ignores the ZT00 command and continues on to the end of the pattern.
Regarding the functionality that you crave for :Consider the ZT command as nothing more than a deprecated feature to provide for older xm/it/mod import formats because in most of those trackers, you could not change the pattern size of individual patterns.
Besides the above fact, consider it also a useful aid during the production process to quickly set a boundary so that you can prevent Renoise from continuing beyond a specific moment.
The render feature in Renoise depends on the range that is selected to be rendered from the sequencer.
“Entire song” means the complete sequence range in the sequencer, nothing less. If you want less, then specify so in the render dialog or limit the size of the last pattern up to the row where you want to have it played, since you can alter the pattern size in Renoise.