I did have it creating the Zip file originally, but then the metadata did not import nearly as well as it does with the HTML2ZIP function.
The ability to call other plugins will certainly make things easier. As usual, I'll be looking forward to the next version!
However, I still have the question: after running a custom plugin, does it re-evaluate what type of file is returned and then run the appropriate built-in plugin? For instance, let's say I take a different approach and use my plugin as a preprocessor when converting from PDB to LRF. My plugin would convert the PDB and return the path to the HTML, will calibre then recognize that it needs to finish by using an HTML to LRF builtin plugin? Or would it still attempt a PDB to LRF conversion with the returned HTML file?