I think the op has a good question, though I doubt there is a good aftermarket solution. The Versys has a dimunitive turn signal indicator...its not big, its not yellow and it makes no noise...kinda looks like the neutral light at a glance.
I have owned bikes with and without self canceling turn-signals. The "use the thumb" response applies to all bikes with or without self canceling, it is not a one or the other thing IMO. Of course you use the thumb, but adding self canceling is MUCH better.
My '82 Yamaha XV920RJ had very good self canceling turn-signals, (measured stearing and time) so what happened to "modern" bikes?. My cynical guess is that perhaps litigation may have done it in....just a guess, no facts to back it up.
Since the Versys does not have self canceling turn-signals I wish it had a more obvious 'turn signal indicator'...that could save your life.