The guide is still incomplete, but you can refer to https://ar14.works/mannequin-docs-customdlc as a start.
Viewing post in Custom Clothing Inkscape Gradients
Yeah, that is in our sights too, just still thinking about how to make the best user experience for that. Since Mannequin is 2D-based, multiple poses require multiple images so the UI has to be able to explain & accommodate that. Adding logo might be the easiest, but other than that we're still weighing our options for best results. Thank you for your feedback!