I had heard the same thing about yields being reduced during flowering so I decided to do an extract on a well known and otherwise consistent species which was in flower. it had previously yielded 1% dmt in the bark.. this time it came out around 0.2%.
there's no concrete answer as to how much it will be reduced.. somehow I doubt it will be
completely devoid, though i suspect it may well be fairly reduced. if you take some 200g or so of phyllodes you'll get a pretty quick idea as they are quite high percentage in alkaloids with the acuminata. if your harvesting sustainably as you said or if you can manage to find a fallen tree or branch then a bunk yield won't matter too much anyway