Digitally Sign SOAP Request using PKI and SSL Cert Created and Sent in an iRule Side Band Connection

I would only add that this is not a current capability, not any specific technical limitation. The Access Policy Manager module actually does do PKI-based digital signatures for SAML requests and responses. Much of what BIG-IP can do today is the result of this forum and user requests. If digital signing is something you'd like to see in iRules, by all means please open a case and ask for it.