Did you read?
Answer is not currently in Beta, and scirra has a beast of a rig with a gigabit connection. (very hard to top, and very expensive to beat)
But answer is NO YOU CAN'T in beta, and beta is going to be around for some time.
And Directly answering your questions....
Is there a way to host my own signalling server, like a local server, or a very own server?
Is the multiplayer plugin will only be available though the official Scirra server?
Once out of beta, there is a possibility.