If you add HATs to R-Pi modules that add additional ports, like USB, audio, and Ethernet ports, how can they be accessed from the Delphi WebCore / Melitus app?
As Miletus is a web app running in a browser embedded in a native shell app and what Miletus exposes to the web app as hardware access on Raspberry Pi is i²c, SPI, GPIO, UART, you need web api's to access anything else for now.
I have not verified how good web USB support is on webkit for Raspberry Pi: https://web.dev/usb/ needs to be checked.
Web audio API should be available : Web Audio API - Web APIs | MDN
For ethernet, there is xmlhttprequest for HTTP access and web sockets.