You are right, majority of NREL written measures takes the inputs (or 'arguments' in scripting world) in IP units. These "arguments" (ex: Supply Temp in F) gets converted into the SI unit systems within the .RB (ruby file) by using the given methods: https://nrel.github.io/OpenStudio-use...
For ex, If you open any measure in notepad++ file (or any text editor) where the inputs were needed in F for temperature, you will find a method similar to "val_in_celsius = OpenStudio.convert(val_in_fahrenheit,"F","C")" to convert the temp F values in Celsius.
As energyPlus runs all the simulations in SI system, we can anyways delete this "conversion method" from the RB file (precisely) & just provide the inputs in SI units to the measure in the argument section. All you need to do is careful editing of your measure RB files which shouldn't be a huge trouble if you are familiar with some of the measure writing stuff. If it's me with 1 out of 10 measure writing experience, I would start with "commenting out" all these "conversion methods" by adding a "#" before the methods.
For ex, change:
val_in_celsius = OpenStudio.convert(val_in_fahrenheit,"F","C")
to
[#]val_in_celsius = OpenStudio.convert(val_in_fahrenheit,"F","C") - remove parenthesis from #
As a test run, try it for the simplest measure & see if you can understand the workflow for what you want.
