Uffe and Simon M, thank you both for your replies. I have found some valuable information in your answers to my questions!
Furthermore, I'm pretty sure that the image in the XMI format is just a bitmap with no reference to the elements it contains, so I don't think you could get a useful HTML page (with clickable elements) from an XMI file.
When the option Generate Diagram Images is selected in the window Publish Model Package, an image is generated (of type .emf, by the way), and the XMI-file contains information about all elements in the diagram, in the form of <element> tags with an attribute geometry. Thus, there actually
is a reference to the elements in the image.
We now plan to apply an XSLT-transformation to the XMI-file, which should result in an HTML-file. This method should give us all options to finetune the result. (It will require some more investigation, but that should be fun!)
Thanks again!