I have upgraded my Zoom installation to version 6 and have been playing with it for a couple of days, and overall I have to say that it is very nice and well worth the upgrade cost.
One of the interesting things has been looking at some of the results generated through stemming, and whilst it does exactly what it says on the tin very well, I wondered if it would be possible/desrable to have it as a selectable option on the search results page; either to 'turn off variations' if th user just wanted boat and not boats etc or as a turnonable option if there were a limited number of search results like the spelling suggestion option.
Anyway just a thought and good upgrade.
There is no option to allow the user to control if stemming is used, or not, in real time. You can turn stemming on or off from the Zoom configuration menu, but this must be done before building the index.
99% of users won't know what "stemming" or "word variations" means. And for the 1% who do understand the significance, only a small fraction of them are going to want to change the settings in real time. So I don't know if it would be a feature that would benefit many people.
Further the structure of the index changes depending on if stemming is on or off. So supporting a real time switch would be technically tricky.