I couldn’t find a custom node for renaming worksets, but it’s possible since 2015 R2 with the WorksetTable.RenameWorkset method.
import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import *
clr.AddReference('RevitNodes')
import Revit
clr.ImportExtensions(Revit.GeometryConversion)
clr.ImportExtensions(Revit.Elements)
clr.AddReference('RevitServices')
import RevitServices
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager
doc = DocumentManager.Instance.CurrentDBDocument
#Converting input from Dynamo to Revit
workset = UnwrapElement(IN[0])
name = IN[1]
#rename workset in a transaction
TransactionManager.Instance.EnsureInTransaction(doc)
WorksetTable.RenameWorkset(doc,workset.Id,name);
TransactionManager.Instance.TransactionTaskDone()
OUT = workset