i´ve found that same registry key over here but i am not sure if this will work reliable if multiple string boxes with different titles are used. RhinoScript (vb) seems to remeber only the last one opened like in below example:
Dim text1, text2, result1, result2
text1 = "Hello World"
text2 = "Hello Rhino"
result1 = Rhino.StringBox("Message1", text1, "Title1")
If Not IsNull(result1) Then Rhino.Print result1
result2 = Rhino.StringBox("Message2", text2, "Title2")
If Not IsNull(result2) Then Rhino.Print result2
For the HtmlBox() i´ve used to write the html file to be read to the temp directory and call it from within a plugin or script without having to mess with the registry.