How much of a Linear B corpus is there? Deciphering Mesopotamian tablets included some experiments in which different people were given the same text to translate, and their agreement on the meaning was taken as evidence that the decipherment was more or less valid.
As long as there's enough Linear B material that we don't need to assume everyone in the field will be familiar with every extant text, it should be fairly straightforward to run that same experiment for Linear B, which would pretty well settle the "the character mapping is so incredibly loose that you can translate Greek from any random collocation of linear B characters" issue.
If you really want to stand behind that wording, you could very easily make up a text ("any random collocation of linear B characters") and see what different people thought it meant.
I don't think that you could get away with it. There's not enough out there. However you'll probably appreciate this similar jest from Michael Stokes (source Douglas Young's "Is Linear B Deciphered" for Arion in 1965).
As long as there's enough Linear B material that we don't need to assume everyone in the field will be familiar with every extant text, it should be fairly straightforward to run that same experiment for Linear B, which would pretty well settle the "the character mapping is so incredibly loose that you can translate Greek from any random collocation of linear B characters" issue.
If you really want to stand behind that wording, you could very easily make up a text ("any random collocation of linear B characters") and see what different people thought it meant.