OK, I thought of something but it requires custom development. You could use the "post data to server" form processing step to send a message to a web service. The service would receive the message, wait a couple of seconds, and then send a message back to Eloqua using the REST API to tell Eloqua to send the confirmation email - see this article: Eloqua REST API - Sending Email
The problem there is how to manage the confirmation email - that will be difficult unless you can use the same confirmation email for all campaigns. You could do that by writing the campaign-specific data to contact fields and using merge fields in the confirmation email to fill it in (that's how we do it). Otherwise you have to give your web service some basis on which to select the email - but that's not that hard, you could do it with a hidden form field because you can pass all of the form field values to the web service in the post data to server step.
So I think this would work, but you'd have to develop a web service.
I suppose another possible approach would be to send a message to a service which would then use the API to determine the campaign steps involved and move the contact to another campaign step. I think you can retrieve and update campaign step membership data via the API. This would avoid the problem of managing the confirmation email, but the service would have to be more complicated.