I know its not as popular as it once was but could you add support for unpacking ACE archives
Mike, your request is described in 7-Zip's FAQ:
Support for ACE can be implemented, if the source code of that format is made available, and if the source code license will allow using that code.
It is not open source but there is a extraction DLL file that the company released under a freeware license for projects like this.
As far as I know, the GPL does not allow distributing libraries without source code. So the UnAceV2.Dll may not be distributed with 7-Zip. Perhaps, it is possible to provide interface for that library, so that if the user downloads it manually, he/she will be able to unpack ACE archives, but I don't know that for sure.