Is it possible to add combo-boxes, option groups and similar items to QGIS processing scripts? The documentation mentions text-boxes ('string', 'number'), file browser etc. I would like to add a combo-box with a list of values - should I make my own Qt from?
If you want to do it the open source way, extend processing with a new parameter type
For development you can just copy the processing plugin into your plugins folder and modify it in a safe environment there. Once finished, please make a pull request so every other processing plugin out there can use this important parameter type as well.