Asterisk Peer Matching and/or changing SDP From: Domain

Hi Group

I have recently connected to a managed SBC provider that will allow connectivity to multiple customers with a single SIP trunk and port to my Asterisk softswitch.
Calls from my softswitch to the SBC are matched on a modified From Domain that is assigned to the particular customer on the managed SBC. I am using chan_sip on Asterisk 13.23.1
So my question is twofold:

  1. Is it possible to change the From: domain from within the dialplan? I know you can change the To: address from the Dial command and used to be able to change the From: address but you appear to not be able to do so now! It seems that the only way to do this is using a separate SIP Peer for each customer and set fromdomain=
  2. Now that I have multiple peers with the same hostname and port, how am I able to determine which peer is matched on incoming INVITES from the SBC? It appears to be the first one in the list in sip.conf. Can this be guaranteed?

Thanks
Mike