It's pretty handy, but doing it for RSA and other crypto systems which rely on large primes is much harder than doing it for crypto systems which can basically use random numbers with a few safety checks just due to the nature of key derivation functions. It is possible though, I seem to remember seeing a paper on it a while back and it should be entirely achievable to do something simple like seed the rng you use for generating RSA keys with an explicit value like that.

For basic signing and verifying purposes I believe Electrum and a few other clients actually include functionality for this which could be used as an alternative to PGP.