<marc(a)uberbox.org>Of course there already exists a way to thank IP
editors. It is to go to their talk page and leave them a message that says
"Thanks for your edit here [link to diff]." It is far more personal, far
more likely to encourage the user to edit further (and maybe create an
account?) based on research on the effects of template versus personalized
talk page messages to new editors, and doesn't require anyone to write any
code whatsoever.
I'm not entirely certain it's a good idea to "technologize" such very
basic
user interactions. It takes as much work to "thank" someone using
notifications as it does to leave them a talk page message.
Risker/Anne