Are there any custom/customer codes that aren't used or are rarely used for the NEC format? I'm trying to make sure remotes don't conflict with each other.

I am not sure if I can really answer your question, however, there should be plenty of possibilities. There are several NEC format variations and many device variations within. The NEC format is widely used so I imagine that there are endless sets in use and many unused. Perhaps another member can speak to specific codes sets with some authority.

I imagine this is for use with the Polk tuners?

Be sure and make the code info very accessable, comprehensive, fully discrete, available in multiple formats in addition to raw data and logical and I am assuming Polk will avoid the trash talk of control systems programers the world 'round :).