Not sure about that, but there are multiple alpha settings for image/frame import. You can specify the background color for the alpha mask which means you should be able to import jpg files with background & apply a bitmap (I think it was called?) mask (like they used to do years ago, or still do in rpgMaker, I think). So I see no reason why you couldn't use a 256 color indexed jpg file.
Don't see much point though as you can further compress all your image files on export/build by converting to webP. I'm not sure if you are allowed to use webP formats by default but anyway: https://developers.google.com/speed/webp/