The documentation for Requirements for NPrinting 16.x.x.x versions only contains the most up-to-date information with regard to your question. Although we hope to see this become supported at some time in the future, the road map does not currently include support for reverse proxy and load balancing solutions. Documentation via release notes and the Qlik Community will be updated accordingly when support for these configurations are added.
The subject is also managed in the following online documentation as well:
For convenience, I have placed the comment directly below for all viewers of this post.
NOTE: NPrinting On Demand does NOT support reverse proxy server configurations nor software/hardware web load balancers at this time. It is currently designed to work with http or https in a non load-balanced front environment where ports 9000 and 9001 are left open for use with NPrinting On Demand Exclusively.