Bert's history of negative CB tests might count for something, but I think not very much. It can always be argued that most (if not all?) of those tests did not make use of the latest technology allowing for lower detection limits. Not to mention that even if Bert had been using CB periodically throughout his career, he might very well have dodged positives just by being careful. This one time, as often happens eventually, he got careless. Surely, we in the clinic have long mocked the "I never tested positive" defense in other cases, why would it count for very much here?
All of us here who have examined the situation have come to the agreement that the only two possibilities are contaminated meat and transfusion. Given that he can't produce this contaminated meat, anything Bert can do to cast doubt on the transfusion scenario is a big plus.
Here is what I would have advised him. Make some rough calculations. Assume he did transfuse in June, and at the time he was taking the minimum dose of CB that could possibly be performance enhancing in some manner, including weight loss. Calculate the amount that could have gotten into his hair follicles, and from there the amount that could be in his hair at specific times later.
Of course these are very rough calculations, but they are better than nothing. If they show that there wouldn't be enough to detect, change the starting assumptions. Assume he was taking a larger dose of CB in June. Maybe that still indicates that his hair sample would be negative. But if the calculations suggest an ambiguous conclusion, go with it. You can then say, if I had been transfusing while I was taking CB--THE ONLY ALTERNATIVE TO MEAT CONTAMINATION--then I likely would have tested detectably in the hair test. That's why you have expensive lawyers. To juggle the figures and suggest the possibility.
And don't forget hair that isn't cut, like pubic hair. That could be tested even now. Make the case. If he really is innocent, what in the world is there to lose? The worst case outcome is that everyone will look at the figures and say, nah, a negative hair test means nothing. But the upside is possibly throwing significant scientific evidence against the possibility of a transfusion.