Adding a tooltip to text that matches a regexp?

[Raffaele Ricciardi <rfflrccrd@gmail.com>]

I would like to add a tooltip to any text that matches a regexp.  The 
buffer content is going to change, thus I can't do it once for all. 
Since such text is going to be highlighted by Font Lock, it seems that 
adding the tooltip in `font-lock-add-keywords` could be a solution.  I 
know that I must set the `help-echo` property of the matching text, like 
this:

     (set-text-properties  (match-beginning 0) (match-end 0)
                           '(help-echo "Tooltip text"
                                       font-lock-face 'my-custom-face))

Can I do this in `font-lock-add-keywords`?  If so, how?  If not, are 
there alternative solutions?

Thank you.