How can I repeat a group of animation phases I am building an animation of a machine where various parts move along paths in turn. So far I have done this without any armatures. I was hoping to string several cycles of operation together, and modify object parameters (using python script) for each machine cycle.

From previous questions, it seems that the NLA editor can't work with groups of actions associated with multiple objects - so maybe the best I can achieve is to write a script to step and repeat the keyframes for all objects? Alternatively, is it possible to render-time transpose my keyframes on the fly at the end of the cycle, or programatically render frames 1..300 twice (with some tweaks) to achieve 1..600?

There are two reasons for wishing to automate the process. Firstly, it would allow me to generate any number of different cycles based on a small number of parameters (my first trial has 5 cycles of 300 frames). Second, as I have already discovered, defects in my base model only become obvious after I made and modified 5 copies of the basic cycle.

(for what its worth, I'm not an artist - just trying to find a good way to demonstrate how the machine works)
Does it need to be programmatic? If not, select one of the objects, select the keyframes (in the graph editor) in frames 1-300: 


Hit Shift+D to duplicate the keyframes
Then hit G and 300 
Then Enter to confirm the operation


This will duplicate the exact keyframe in 1-300 and move the duplicates to 301-600.

If it is more complicated than that, perhaps you might update your question with more details on exactly what you are attempting to accomplish.