There's a nice little utility called sfo which works very well in concert with 7z— one can quickly verify whether an ISO contains PSP_GAME\PARAM.SFO, extract it, query/modify it, and then update the image.
None of that is possible with a .CSO image without first decompressing the entire thing, often times from an HDD. It would be super useful to at least be able list the files within an image before deciding whether to decompress.
I have no idea how tall of an order this is in terms of complexity, and if Stdin/Stdout would be easier to implement, that would be an equally useful feature.