Setting the builtin parameter accessible thru the family editor?

[Konrad_K_Sobon]

This should do the trick:

#Copyright(c) 2017, Konrad K Sobon
# @arch_laboratory, http://archi-lab.net

# Import Element wrapper extension methods
import clr
clr.AddReference("RevitNodes")
import Revit
clr.ImportExtensions(Revit.Elements)

# Import DocumentManager and TransactionManager
clr.AddReference("RevitServices")
import RevitServices
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager
doc = DocumentManager.Instance.CurrentDBDocument

# Import RevitAPI
clr.AddReference("RevitAPI")
import Autodesk
from Autodesk.Revit.DB import *
import System

import sys
pyt_path = r'C:\Program Files (x86)\IronPython 2.7\Lib'
sys.path.append(pyt_path)

paramName = IN[1]
value = IN[2]

class FamilyOption(IFamilyLoadOptions):
	def OnFamilyFound(self, familyInUse, overwriteParameterValues):
		overwriteParameterValues = True
		return True

	def OnSharedFamilyFound(self, sharedFamily, familyInUse, source, overwriteParameterValues):
		return True

def GetBuiltInParam(paramName):
	builtInParams = System.Enum.GetValues(BuiltInParameter)
	test = []
	for i in builtInParams:
		if i.ToString() == paramName:
			test.append(i)
			break
		else:
			continue
	if len(test) == 0:
		return None
	else:
		return test[0]

try:
	errorReport = None
	output = []
	uniqueFams = set()
	bip = GetBuiltInParam(paramName)
	for i in IN[0]:
		family = UnwrapElement(i).Symbol.Family
		if family not in uniqueFams:
			uniqueFams.add(family)
			famDoc = doc.EditFamily(family)
			if famDoc != None:
				ownerFamily = famDoc.OwnerFamily
				param = ownerFamily.get_Parameter(bip)
				TransactionManager.Instance.EnsureInTransaction(famDoc)
				param.Set(value)
				TransactionManager.Instance.TransactionTaskDone()
				output.append(famDoc.LoadFamily(doc, FamilyOption()))
except:
	# if error accurs anywhere in the process catch it
	import traceback
	errorReport = traceback.format_exc()

#Assign your output to the OUT variable
if errorReport == None:
	OUT = output
else:
	OUT = errorReport