if i understand correctly,
B4X:
webview.Color = Colors.Transparent
gives you a transparent webview. (does for me)
if i load an image and set it as activity background, and initialize a webview with the transparent background (and add some html to the webview), it behaves just like a transparent canvas: i see the text as if it were on the image. is that what you're talking about?
ugly (but illustrative) example attached