Extracting SSL Certificate Issuer from Server Side Connection

Well no. SERVER_DATA is a server-side event and typically triggers with a TCP::collect in the SERVER_CONNECTED event, which itself is triggered on a server side (node to BIG-IP) TCP handshake. The issue here is that you're looking for traffic coming from the BIG-IP, and aside for a few exceptions, there aren't iRule events for traffic leaving the BIG-IP. In other words, anything you do in SERVER_DATA is relative to traffic coming from the server, which in this case is the remote Internet host.

The only option you really have for capturing client-side traffic coming from the BIG-IP to the client is to layer a VIP in front of this VIP. This external layered VIP wouldn't do any SSL, just simple TCP pass-through.

when CLIENT_ACCEPTED {
    virtual internal_vip
}

But it would see all of the TLS traffic coming from the SSLFWD configuration and could extract that information, albeit with TCP binary iRules processing.