Back in March, I opined that an S2K bit was out of place, noting that the S2K itself isn't broken, and that it is used in other contexts.
Hal Finney offered the following alternative, which I like much better than tweaking the S2K itself:
Another place we could represent the alternative format is the byte which comes shortly before the S2K in the secret key packet. This byte is fixed at a value of 255 to flag that an S2K is in use. We could perhaps use some alternate value for this byte to flag that the private key is using a different form of checksum protection.
Perhaps a value of 254?
On a slightly related note, could we also add placeholders to the spec for the NAI-specific things that have come into practice? One example is the S2K bits for raw and split keys, which is why it came to mind now. Hal mentioned an X.509 certificate signature subpacket, and a CRL packet type. The PhotoID packet is yet another that was discussed recently.