It's great to see that someone is trying to establish standards in bitcoin community. A while ago I was doing a bit of research regarding the private key export and came to a conclusion that OpenPGP message format would suit this use case perfectly. I encourage you to take look into RFC 4880 if you haven't already, quite a brilliant piece of work. What I advocate is to use the existing, vast system that OpenPGP is and try fitting bitcoin into it.

Advantages to taking this approach are numerous.

• OpenPGP is a well known and tested standard
• Security is built in
• There is room for growth and modifications
• It's very robust and covers almost all use cases
• Possible use of existing OpenPGP compliant architecture

There is one great disadvantage, though. With robustness comes complexity - it's not a quick and easy fix to the problem of key sharing but rather a lengthy process of integrating bitcoin into OpenPGP ecosystem.