I like the "plaster was used to move heavy blocks idea".. Today it would be called "slurry".
And if the chamber were then sealed there would be no opportunity to clean it up .. Kinda like cement oozing out from between bricks inside a double cavity wall..
I can't see how, or even any reason why the white wash would be done at any other time but in situ.
So can anyone think why or how the interior of the chamber could have been white washed before construction. By "before construction" I mean at any time before the last block was installed in each chamber. Thus preventing further access.