In an InputScript I capture an SAP error message and then display it with my own Message command. But unlike standard SAP statusline, the user cannot double click on it to get more information. Is there a way to achieve this?

It is possible to display your own text with the following technique:

GuiXT Script: 

Message "my special message" -statusline

On "/messagehelp" process="mymsgdoc.txt"

InputScript "mymsgdoc.txt":

Message "This is the first line of the msg explanation\nAnd this is the second line" title="MSG Help"