Can't create custom Workflow Task because getTemplatePath is hardcoded!
I was thinking about creating a new Extension module which adds in a new Workflow Task.
In the com_vtiger_workflow_tasktypes table is a column for the path to the template. But it looks like this is completely ignored by the function getTemplatePath() in the Settings_Workflows_TaskType_Model Class.
Meaning, I think, it is not possible to do this without writing a custom template to layouts/vlayout/modules/Settings/Workflows/Tasks/