I have a lot of cc I'd like to make available in all CAS categories but it would be too tedious to do it by hand using s3pe. So, I was wondering if there's any tool that would allow me to select multiple package files and batch edit them, so that I won't have to do them one by one. Does anyone know of any such program?

Batch Resource Editor does just that. Alternatively, if you install NRaas MasterController with the MC Integration module, you can simply disable clothing filters for certain CAS categories.

Ok, Batch Resource Editor looks very promising! It seems I just need to create an XML file as per the example in the downloaded zip folder, which shouldn't be too difficult. Just a quick question though -- if I don't check the box that says "modify custom content directly", the program will create an override mod for my specified cc, which I'll have to put into my Overrides folder, right? I'm asking, because in the download page you're strongly recommending to not check it the first time.

Yes, this will create a mod that you'll need to place in Overrides. I only recommend leaving this option disabled at first so that, in case of a mistake in your XML file, the tool doesn't mess up your actual CC files. You can check the created mod in S3PE, and if everything looks good, then you can edit your CC directly. Let me know if you need help creating an XML file.

Thanks for the clarification. I haven't tried it in-game but the XML I created as a test seemed to have worked fine so far. I'll let you know if there's any problems!

By the way, do you know if there's any easy or automated ways to get the instance ID of the individual CASP resources inside a package file? Again, opening each one with S3PE would be too tedious XD

You don't actually need to get instance IDs for batch editing. If you don't use the GUID node in your XML file, the tool will interpret this as if you want to edit all resources in the selected paths. Simply place all the CC files you want to edit in a new folder, specify that folder as the Custom Content path, and ensure that only that path is checked in the tool. If you have a merged .package file with multiple CASPs and want to edit only specific ones, select and export them to a separate .package file and place that file in the specified folder. After editing the file with the tool, import it back into the merged package (replacing existing resources) or use it as an override mod.

Ok, thanks!