What I've typically done for New Construction of residential buildings (mostly mid/high rise) is to follow the guidelines and calculator given by the ENERGY STAR Multifamily High Rise (MFHR) program. Basically, based on population classification it gives you how many gallons of hot water per day and per occupant. Multiply that by the number of occupants and you'll get your baseline usage.
The tool spits out a peak flow rate and a fractional schedule to use with it, so you just have to create these in OpenStudio.
Note that for the water fixture definition object, as far as I know these values are for a target water temperature of 120F.
You can download the Performance Path Calculator on this page.
You should also take a look at the ASHRAE Handbook of Fundamentals - HVAC Applications (2015, 2011, 2007...), chapter called "Service Water Heating". It has load profiles for different building types and a couple of different ways to size your heater.
This is a good feature request, so if you want do add water per person and or per area, you an addd a request on user voice. This could be a feature only to OpenStudio (and it would hard-size it on forward translation) or it could be implemented in EnergyPlus as well. For stop gap could write measures applied at run time to mimic this behavior.