This answer mentions that a plausible in-universe reason for not talking to Dumbledore's portrait in Deathly Hallows was that the school was under Snape/DeathEater control. Also, they had to use Aberforth's tunnel to get to the school in the end.
They could have considered using the same method that Malfoy used to bring DE's to the school at the end of HBP - The Vanishing Cabinets.
So, is there any canon explanation of what happened to both Vanishing Cabinets (one in the Room of Requirement, one in... wherever) after HBP?
I'm not asking whether this would be a good idea (probably bad - DEs knew about the cabinets and would have guarded them, hopefully).