Is there a way to manipulate SAMLRequest in an iRule

Sadly, I don't believe deflation/inflation are supported processes in iRules. You can definitely crack open a base64-encoded SAML POST request and muck with its values, but there's no way (in iRules) to deal with a deflated SAML Request message. I suppose, technically, it might be possible to create a pair of "service" VIPs and iRules to inflate the request, allow you to alter it, and then deflate it again, but this wouldn't be the most trivial thing to accomplish.