No, you cannot - unless an undocumented parameter has been added.
http://www.mikrotik.com/testdocs/ros/3.0/pnp/hotspot.php - this is unfortunately not on the wiki yet. Look at the section about customizing HTTP servlet pages, it contains valid variables that you can use, there is no access to the comment field. It’s unlikely this can be made accessible as the Hotspot pages should be agnostic on whether the user is stored in the local database or a RADIUS server that may not have a corresponding field.