An idle thought I had about the file shelter code a while back: It should be possible to make a single-file self installer that has all the files in "file shelter" format built into the binary. Since you can upload files to root of the card with standard PTP operations, the install could be as simple as: Run some app or command on your PC, run firmware update on the cam. The fi2 file extracts the real CHDK install, makes the card bootable, and deletes itself.
However, there's enough caveats that it might just add confusion
1) It can't handle formatting / partitioning, so the FAT32 / exFAT limits would still apply.
2) While you can technically upload with regular PTP, convincing normal software to do it may be difficult or impossible. On windows, requiring the user to install libusb and chkdptp would defeat the purpose.
3) Some cameras crash if A/ is not present in the upload filename, while the rest crash if it is. These aren't currently identified, although the sig finder could probably detect it based on the assert.