Ok, I am not sure if my solution really is the best one, or if I simply doing something wrong, but I could not figure out how to set the "host-path" for the DreamHost instance launched by the HttpHandler.
I checked the source and I thing it is simply a matter of adding one line to the source. It might be just a hack, but at least this way it is working for me 
I replaced the following code in mindtouch.core > http > httphandler.cs
Code:
XDoc config = new XDoc("config")
.Elem("service-dir", ConfigurationSettings.AppSettings["service-dir"])
.Elem("apikey", apikey);
with
Code:
XDoc config = new XDoc("config")
.Elem("service-dir", ConfigurationSettings.AppSettings["service-dir"])
.Elem("host-path", ConfigurationSettings.AppSettings["host-path"])
.Elem("apikey", apikey);
This way the "host-path" configuration is read from the webconfig appSettings section and passed onto the launched DreamHost process.