Joey is correct, the SMP can’t use the .png to .jpg trick. However, this is only necessary if you need to display coverart with transparency, such as with coverart that has rounded corners or reflections.
Although you can’t do this with the SMP I have made a workaround for my Darklght theme that does mimic reflections and rounded corners using standard .jpg images which you can see here:
Also, as Joey said, you can keep the xml metadata from being displayed by editing the gallery view xml. Or if this is what you are wanting to do DeVicious has her Black Mamba theme that uses this method already. It is developed for the HUB not the SMP, but it should still work. However, with this method you should expect some delay between when you scoll to a video and when the moviesheet is displayed, so you will have to wait for the metadata to be displayed when the moviesheet is.
Also, as Joey said, there is no custom firmware for the HUB or SMP. The custom firmware that you are speaking of is on for the older WDTV Live (Gen1) and WDTV Live Plus (Gen 2) players. And the people who developed the custom firmware for these 2 players have said that they are not going even attempt to make custom firmware for the HUB & SMP, so there will probably never be any.