I think using a hash of the key as the key selector is a fine idea.
PGP has a key selector, called the keyID, which is currently the
low-order 64 bits of the modulus. PGP also has a key fingerprint (the
MD5 of the key) that people use to verify the keys over the phone. For
over a year now, I have been planning to consolidate them, with a keyid
as the hash of the key. I planned to use the SHA hash to do it. A
message has a keyID prefixed to it to select which private key to use
to decrypt the message, and I plan to make that new keyid prefix a
variable-length left subset of the SHA, the length of which is a
configuration parameter, from zero to 160 bits. Perhaps the default
length would be 64 bits.
I wonder if there exists the possiblity of making the hash be computed
the same way for both PEM and PGP keys. This would make the keyID the
same for both schemes. Just a thought.
prz