As indicated above, you would have to brute force your way through 10^77 different combinations to guarantee a successful collision.

You haven't shown that a SHA2 collision is needed. SHA2 get's used to create the message digest. That in turn get's encrypted via the private key.

You are referring to several traits which are common to almost all cryptographic systems, yet you are implying these are traits unique to PGP.

No, I haven't said anything about something being unique to PGP. My core claim is that you haven't demostrated safety.

The standard cryptopunk way of thinking is to mistrust statements about crypto that aren't demostrated to be true.

You can manipulate the output of (or "add specific entropy to") any hash function. It is, however, absurdly difficult to convey meaningful information in the output of a hash function. See above regarding the amount of work required.

SHA2 in particular hasn't been successfully attacked. Other hash functions on the other hand have been.

I remember sitting in the audience at the CCC while they explained how the broke SSL at the time via a collision and some other tricks.

and anyone unfamiliar with cryptography will likely be confused and mislead.

They will see that you haven't demostrated what you claimed in a clear fashion. Anyone who wants safety via crypto-systems should use systems that who's safety is well established. It's as it should be.

Furthermore, because a private key is longer than a PGP signature, it is literally impossible to encode the key in the signature.

It doesn't have to get out all the information in one swoop if you use the thing multiple times. The fact that you make that argument suggests that you don't have a real argument that demonstrates safety.