Hello,
I think it would be a great killer app if we could off-load the secret key onto the Yubikey NEO. This can be done by generating an ephemeral key given a user pin/password and storing it onto the NEO or by using the OpenPGP applet included in the device.
By doing so Synchthing wouldn’t need to store the key ever on the hard-drive / ram.
For info on the Yubikey NEO: you can check Yubico website, they have a github repository github.com/yubico and opensource.yubico.com website.
Regards, GuruMeditation.