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?