I have a project with a very large number of read-only text boxes. How can I enable the user to copy the text inside any box to clipboard when long clicked? Can this be done through a single event?
when any textbox.long clicked
if textbox read only = on
copy textbox.text to clipboard 
if the textbox is read only, why not use a label?
i know there is a extension can make the label selectable.
You can try the clipboard tool extension by Taifun. and of course I agree with Kevinkun's point—why should you use a read-only textbox instead of labels?
Investigate the Sharing component.
It has a block that can be fed text for sharing with other apps like text editors or spreadsheets.
I can't tell the difference between that and a clipboard.