I've just pushed a big update to the Lua extension system in Marasi (an open source application security testing proxy). It includes two main updates:
1) Addition of modules that you would want to use directly during your testing (think encoding, encryption, request / response manipulation)
2) Ability to extend the GUI of the application. This was challenging as Marasi the proxy is meant to be used as a library and the GUI is a separate application. Extensions run on the proxy and not the GUI application. Right now I've settled on having the GUI inject its specific render and update methods to handle that bridge.
You can check it out at https://marasi.app or the repo directly (https://github.com/tfkr-ae/marasi-app) but I would recommend download the latest development release as it includes a fix for the UI locking up of there is a lot of traffic going through the proxy.