Hello pbourke,
Currently there's no option to create the UV parameterization manually in Metashape. External tools that are commonly used are Blender, Maya, ZBrush. After that the mesh should be imported back to Metashape and "Keep UV" option should be selected for mapping mode option.
If you are experiencing the inefficient parametrization for your models, please provide a few sample mesh models to us for testing to support@agisoft.com.