So, the only hard part of spanning CDs is figuring out where to cut the files so that they don't go over the disc size. This is what Inno is doing automatically that IJ does not. In the current versions, it's impossible to jump into the middle of a build and do any of this kind of work, but this could be easily done if you do the work yourself.
Meaning that if you create multiple file groups and add files to them and make sure that each group doesn't go over your disc size, the builder will just build the separate archives as you've designed them and not care about size. If you do that work upfront, all we need to do during installation is mount each archive and install them one-by-one.
It should be possible to do. Almost anything is possible in IJ with enough scripting. 0-] I'll help you out with getting things working if you're willing to put in the work. First things first, build a project and split all your files into file groups that are less than the size of a disc. Once we have that, I can take a look at your project and figure out exactly how to do the rest.
It might take a few tweaks to the core, but they should be fairly simple. All the pieces are there in the installer, it's just the builder that lacks the pieces to do what you want automatically. 0-]