If you are going the way of includes to accomplish this, please consider intergrating and allowing us to choose between php or SSI. I would prefer these so that you could in theory use any method to edit the one header file on the server later on (example: File Manager, FTP, or SSH editor) if it's a quick edit rather than having to return to RSD and republish the whole site again).

i.e. I would not like to see this implemented in such a way that the RSD maintains a single header file, but publishes only the assembled pages such that you still have to edit 20 separate pages on the server.