I would very much like to disable VO2max detection for trail running activities too. When I owned Garmin, my VO2max continuously declining because a do at least 1-2 serious trail runs per week on a steep terrain. Once I disabled trail running VO2max on Garmin, the estimate steadily went up from 47 to over 52 in just a few months.
Now, that I have owned Suunto Race S and S for over a year, my VO2max has been stuck in the 48-49 range, occasionally going to upper 49. I feel that is a bit too low. Every time I do a trail run, it always goes down to lower 48 or even upper 47. The funny thing is that I am a relatively better trail runner than I am a road runner. I have good and efficient technique when running on technical trails, but despite that the VO2max estimate is always punished!