Some of the details of adding new control types was posted in the other thread, but I'd really like it to be class based with dynamic dispatch. At the moment it is a bit of a pain to add new types but clearly is possible.
Start by looking in the Web Front Panel_class\Private\Object Factory.vi. This VI is what identifies each object and figures out the type that it is. The Init state looks at the control ClassName, then goes to other states based on that. You can probably just duplicate the String class and do what it does (which isn't much) until it goes to the Finish state. Here we create the new object class. Here I think you can go into the Select object class type.
Now how do we update the object we just made? Open the Front Panel Objects\Select_class\Get JSON Data.vi and edit it to support the combo box class type. This VI at the moment assumes the data is always a numeric, so you'll need to do some type checking, or maybe attempt both the variant to data on string and numeric and use the one that doesn't generate an error. Or feel really adventurous and update the Front Panel Objects\Select_class\Attributes.ctl to have the added attribute of Numeric, or String which can be set in the Object Factory. If you do add this to the cluster, be sure and update the value to numeric when a ring, or enum are used.
Then the last thing is to look into what to do when the user changes a combo box control value. Update the Front Panel Objects\Select_class\Set JSON Data.vi and set the combo box with new control values. It will return a numeric of which item is selected, so you'll need to look up the list of available items from the combo box and set it. Again if you update the Attribute type def you'll know to treat it as a string or numeric. Otherwise you can look at the data type of the control, or the class the control belongs to. But since this will be called a bunch of times the more efficient method might be to update that type def to contain things like what the data type or ClassName is. This will have a limitation that a combo box can't have a value that is undefined or not in the table. Not sure what would be needed to support that but it would probably involve looking into the web control support.
If you give more information on the other two controls that aren't being updated I can try to help. Are the controls just normal string? If they are images do they update? I realize you can't control them then, I'm just trying to figure out what it could be