Hi Long -
thanks for the reply. I found this on Codepen - https://codepen.io/yguo/pen/OyYGxQ
it uses HTML, JS, and a little CSS to create a signature field. I think its a good start to investigate a custom implementation, but it would definitely be better if either MB provided this as a dedicated field type.
But I'm also exploring making my own custom MG field type.
Please let me/us know how the team responds to this request.