Is it safe to publicly expose PBE-protected PGP secret keys? I am working on a PGP scheme where a private key is created offline (and in volatile memory) and must be reused on very rare occasions to sign a piece of exportable data.  The private key is locked using a random passphrase with ~256 bits of entropy, and the passphrase and resultant PGP secret key are written on paper and locked up for later use.  The key needs to remain usable for years in the future.

To simplify the future import procedure by reducing the amount of manually entered data, would it be safe to publicly export the secret key file provided the passphrase is kept locked up?  That way, the secret key can be imported off of a flash drive or similar, while only the passphrase needs to be keyed in.
What I understand from the question, as if you have a Certificate Authority key around. 

Using an air-gapped pc, raspberry pi zero, etc. is a good choice. The important points;


Sign the document in the offline solution, do not carry the key.
You can have more than one air-gapped solution. Extra hardware against failures.
You can keep your old paper around.
Have an extra security against the easy copying from the device




Extream care 

Off-line encryption: In the military, when a message is top secret, the message, firstly, encrypted with an off-line system that has no connection to the network. After the off-line encryption, the message is sent by on-line crypto. The problem is not finished here. 

Tempest Attack: The signal from a pc can be carried by air or the power lines. This requires an expert in this area to filter the machine. So the off-line system and the power lines are filtered.

Covert channel: If an adversary can install a software to your air gapped solution that he can use the heat to transfer the data. Have a mechanism to prevent access to the solution.