"Spontaneous bruising is the major clinical sign."
I am thinking that this is actually a "bruise", and maybe because of her size and the area it is (the knee where the skin is stretched as she sits) maybe it opened? Maybe Vet will have a thought? I will be asking my vet when I see her later, he did say that it wasn't done in transport or when she was up and moving. That was my first thought. Moving big dogs can be dangerous to the dog and to the staff. When they are knocked out they aren't as easy to move as a person. Some vets won't do some surgeries on giant dogs because of the strength needed and the chance of harm to everyone. At least I have had one vet tell me that. The woman who did Emmett's neuter wouldn't do Addie's spay even though Addie is 40pounds lighter. She said it was just too much to move them and dangerous. She did also have some arthritis in her wrists which also made working on a giant dog harder on her.
Yeah,its most likely a bruise----its hard to say without seeing it, but consider the cage is probably metal (or coated wood) and even with a pad in it, if she rolls across the bone just the right way, considering her current condition, she's in a condition to develop these type of "cage sores". Typically they start as a bad bruise, then the dog starts licking them, making them appear worse.
To answer your other questions.... I doubt it if its anesthesia drug related. I have no way of knowing for sure without knowing exactly what drug protocol they used (there are several considered "standard" for large breed dogs). Thrombocytopenia is more associated with antibiotics and similar pharmaceuticals, not anesthesia drugs.
Yes, this may be a lifelong condition. Right now she's going through a bad spell. She's got to get through this. Events at a later time could trigger a reoccurance.