I know canvases can be cloned once signed by combining them with a same-size canvas in a crafting interface, but could we also have an option to have that for paintings that are not signed yet?

It would be useful for a variety of reasons:

• On could make "templates" of sorts (like making a frame texture that you can then reuse for different paintings)
• It would be useful for making variants of an existing painting (like having a portrait but the eyes look the other way etc.) before signing them
• Could also be used for making a "backup" in case you wanna try some changes to the painting and several minutes later you notice you aren't happy with the change.

It might be not what you actually asked for, but you can export your canvas as a file with a command in a chat /paintexport <filename> and then import it to empty canvas /paintimport <filename>

For backup here is conversion tool that I made, it's allowing you to convert exported files to PNG from in-game .paint format.
https://bondarenkoartur.github.io/JoyOfPainting/