Primary subkey subpacket

Hi!

Recently I stumbled over a problems with multiple subkeys. I know
PGP doesn't let the user choose the key at all and GPG uses the
newest key by default. What about a "primary subkey" subpacket
which is placed on the self signature to force the implementation
to use a special subkey. The format should be similar to the 
"primary user id" packet.


        Timo