At some point a line needs to be drawn. I’m weary of expanding the HTTP Actions to the point where every possible combination of headers and bodies can be sent. I’d be more in favor of making a full HTTP library available to the Rules, something better than the default Java HTTP libraries. By the time we will be done enhancing the Actions we will have built our own library anyway so let’s just skip that and let someone else do the work.
But since that isn’t possible right now, you can use the default Java HTTP libraries. You can see some examples OAuth2 using just OH Rules and myopenhab.org and iCloud device data integration in openHAB. Using these libraries you can craft any HTTP command with any headers or body that you need. But the code will be lengthy and ugly compared to what we usually have available to us in Rules.