Absent any reply on this topic I thought to investigate this a bit further by going to the Nuvoton website, where there are a large number of Nuvoton “development tool” boards using Cortex M0, M4, M23 and 8051-compatible processors in both “tiny” and “maker” (Arduino-UNO) style form factors. On the Tiny-style boards they typically provide break-off USB adaptor (“Nu-Link”) for programming. There are also links to software progamming and debugging applications as well.
You can start from the Nuvoton home page. Check out the complete list of Development Boards, as well their Software Development Tool page. There seems to be quite good documentation for both hardware and software available as PDFs from the site.
Since it looks like the primary way to connect to these boards is via USB, I’m not sure how one would use their development environment with a Pimoroni IO Expander (help, Pimoroni?), but a bare board and their develpment environment might be a start. It looks to be a world of its own to explore…