How can I send $player on $serv1 a login packet from $serv2? I think it could be possible using sockets?!
You could, but why would you want to? What would be the purpose of sending the login packet between servers? Do you want to parse it's data?
Proxy's aren't easy to use for everyone. Sending a TransferPacket could be easier for someone. As for sending a login packet from another server, that is more difficult than a TransferPacket, it would require multi-server communication. I don't believe this could be achieved at the current state of MCPE or PMMP, when you send a TransferPacket the client (MCPE) disconnects from the current server and attempts to connect to another server, when you disconnect a client they aren't expecting another login packet; It would most likely be ignored by the client. I'm now curious and I'll test this and see if it's achievable and if so how effective it is.
You're all forgetting one vital thing here, the LoginPacket is only sent from the client. You could try sending a start game packet or play around with some other packets to see what the client responds with.
Just wait for MCPE updates that improve it. MCPE is laggy nowadays anyway, taking several seconds even just to show the settings screen.