Currently we are doing this. I believe support and minor, major all updates should be free if you go down to the subscription route, otherwise it's just ripping of users :)
We are currently offering 1 year subscription which includes free support and all minor, major updates. We added a simple auto-update feature as well to emphasize this. Also frequent updates will keep them happy, so just like SaaS.
Although I can't tell you what worked or not as we just started about a month ago to sell but so far no one really complained.
P.S.: What we do is a niche area and all our competitors are doing this as well. So it's kind of accepted way to do it in this business, like AV business. Where updates generally cost money after a point. So you might hit some extra resistance in your case.