Super, that gave me what I needed to find some relevant information. The standard is defined primarily in RFC 2440 and RFC 3156, and the relevant parts here are in RFC 2440 5.2.3.6:

5.2.3.6. Preferred symmetric algorithms

   (sequence of one-octet values)

   Symmetric algorithm numbers that indicate which algorithms the key
   holder prefers to use.  The subpacket body is an ordered list of
   octets with the most preferred listed first. It is assumed that only
   algorithms listed are supported by the recipient's software.
   Algorithm numbers in section 9. This is only found on a self-
   signature.

9.2. Symmetric Key Algorithms

       ID           Algorithm
       --           ---------
       0          - Plaintext or unencrypted data
       1          - IDEA [IDEA]
       2          - Triple-DES (DES-EDE, as per spec -
                    168 bit key derived from 192)
       3          - CAST5 (128 bit key, as per RFC 2144)
       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
       5          - SAFER-SK128 (13 rounds) [SAFER]
       6          - Reserved for DES/SK
       7          - Reserved for AES with 128-bit key
       8          - Reserved for AES with 192-bit key
       9          - Reserved for AES with 256-bit key
       100 to 110 - Private/Experimental algorithm.

   Implementations MUST implement Triple-DES. Implementations SHOULD
   implement IDEA and CAST5.Implementations MAY implement any other
   algorithm.

Hopefully that should be enough for me to delve into the Crypt::OpenPGP::* innards, and eventually come up with a proposal to the author for an extension to the interface.

Thanks,

Hugo

[Update] second quote was truncated.