Unfortunately, Rebex HTTPS does not support HTTP 1.0. HTTP 1.0 is not very useful today due to its lack of support for virtual hosting (running multiple websites on a single IP address), and we think that adding support for it now, more than 23 years after the introduction of HTTP 1.1, would not be cost-effective for us.
However, in your scenario, where you are working with simplified variants of HTTP anyway, it seems that it might be sufficient to just make it possible to configure Rebex WebClient to announce "HTTP/1.0" in its requests instead of "HTTP/1.1". If you think this would address your requirements, please let me know - we can add support for that.