They are other ways to parametrize your windows outside of the OpenStudio (OS) ecosystem.

Allow me to suggest three things:

• Take a look at JEPlus. You'll find an example most interesting in the example section: 9.3 Change Window to Wall Ratio. Note that if in the first place you can generate a geometry - using OS or Design Building - with a fixed WWR of say 0.1 in the first place, it might be easier to just apply a window multiplier.
• If you have an IDF file for which you want to modify the geometry, you could actually do this using the OpenStudio API and a couple of extra steps. Importing existing IDF geometry in OpenStudio will work nicely, you can make the change and export that. The only problem is that on import from IDF to OpenStudio, it'll strip your HVAC and most of your schedules etc. Basically what you could do is to just replace the geometry, preserving the rest of your IDF file. This shouldn't be that hard.
• You could also use Eppy (GitHub) to do that, it'll allow you to get the vertices of the parent surface pretty easily and then calculate the vertices of your window. If you end up with windows that aren't rectangular, it'll get much trickier than using the OS API though.

In addition to what Julien suggested, you can also use the ParametricPreprocessor that comes with EnergyPlus to modify the coordinates on the windows to achieve the sensitivity to Window to Wall Ratio. See the documentation at:

http://bigladdersoftware.com/epx/docs...

and the example files:

• 1ZoneParameterAspect.idf
• LBuildingAppGRotPar.idf
• ParametricInsulation-5ZoneAirCooled.idf

The first does some vertex manipulation so it is probably the place to start.