Hi Karsten,
First question: why are you signing the form? Is Full Trust required? Full Trust is not a best practice and there are only a small percentage of forms that truly need to be full trust. Domain Trust should be sufficient in 95% of cases. If you can remove Full Trust, that will save you a step.
If you are always publishing to the same location, try just saving. Publish fixes up the manifest.xsf and template.xml to refer to the location, but if you are just saving back to same location (and the promoted properties didn't change), save should be good enough. Save can be done through the file system.
Other options would be to look at using a Visual Studio plugin or using SharePoint commands via the cmdline. But, that will require someone sitting down and writing some code.
We have a tool for automatically deploying a form to SharePoint. We are trying to resuscitate the tool since there have been a number of recent requests. If you are interested in this cmd line tool, please send e-mail to Sales at Qdabra dot com.
Thanks!