I'm still searching for a secure cross-platform solution that I can implement. Private Keys (if stored) must be encrypted and only readable by the clients, maybe with one or more master passwords for different 'key groups'.

SteemConnect doesn't store the keys but it uses delegated key privileges, which leads to a few Apps having active key privileges for thousands of accounts.

Keychain might really be a better solution but I haven't looked into it in detail yet. Will do soon ;)