Hi Marco,
I'm not quite shure how to do this in Jetty configuration which OpenHAB ist using - Jetty docs may help here but these changes will be overwitten by a new OpenHab version. How about that: If you are able to use an own domain conjunction with a dynamic dns service, you could redirect a subdomain an proxy the root context:
2) Use dynamic DNS to redirect this subdomain to your dynamic IP (I suppose you're using such thing)
3) Simply proxy to your internal openHAB box
As of today the most web space providers allow you to create a sufficient amount of subdomains in a registered domain of yours and the big players support dynamic DNS with no additional charge.
regards
Robert