Unfortunately, there is no such feature.
As workaround, you can indeed export to 'HTML ImageMap'. If the setting 'Export Description as Tooltip' in the export dialog is checked, the description is saved as plain text in the HTML file. Another option would be to use the regular save to GraphML. GraphML is an XML format which contains the descriptions in plain text, too, but also all other data of nodes and edges, e.g. label texts, URL, and all custom properties. With basic knowledge of any scripting or programming language, it should be rather easy to extract the desired information from this format.