HXEmu is a Epson HX-20 emulator designed to support most if not all features of the HX-20, including periphereals as well as some special features.
It will be released as open-source under a not yet specified license and it's currently being written in the Pike programming language, but a C++ version is also planned.
The HD6301 chip is part of a hardware emulation library so it can be used in other projects, although it's still under heavy development.
Various pictures on the HX-20 and the HX-PIA (Periphereal Interface Adapter) are available here.

Status: Implementing 6301 opcodes