If they do decide to institute such a rule (and as Python notes, there is substantial opposition to it within WADA; it will only result from a lot of heated debate), it will probably be a level higher than Bert’s, based on the fact that several athletes with higher levels have had their contaminated meat stories accepted. But remember it is proposed only as a level above which no such excuses would be accepted. The converse, that any concentration below that level would be considered not doping, is apparently not on the table here. Which IMO is a good idea, because it recognizes the obvious fact that any concentration, no matter how low, could in principle result from doping.
In this case, it wouldn't necessarily help Bert that much. In fact, all it would do, really, is justify his having any case at all. It would imply that he wasn't automatically suspended, without any recourse to the particular facts of his situation. But that is already where he stands, anyway. He isn't disputing that he tested positive, so he would have no basis at all for his case if there weren't already a de facto level of CB which could be considered resulting from contamination.
The more important question as I see it is whether this kind of level could be fair, or whether it could wrongly convict a rider. As someone noted here, it's possible to register very high levels of CB from eating contaminated meat, much higher than Bert's. But this is extremely rare, even in countries where there is no meat inspection. A study I discussed on an earlier thread, in which meat bought on the street in Mexico was tested, found that about 85% of the samples had CB at levels above the EU standard (0.1 ug/kg). OTOH, most of the samples, about 90%, had less than 2 ug CB/kg.This is 20x the EU standard, but one would probably have to eat at least 200 g, almost half a pound, to get Bert’s level.
The highest level detected in this study was 6 ug/kg. If you ate a large helping of this meat, say, 300 g, you would ingest nearly 2 ug of CB, and your peak urine level would probably be on the order of 200 - 500 pg/ml. So a limit set around there would very unlikely be exceeded by someone through eating contaminated meat.
In food poisoning epidemics, much higher levels have been recorded, as much as 1-1.5 mg/kg. But these incidents are fairly rare. As discussed previously, there are limits to how much CB you can give cattle without killing them (and probably limits to the effectiveness of the drug, also), and even ranchers that push these limits can let much of the CB clear before slaughter, without losing the benefits of the drug.
I do think, though, that any approach to CB should take into account whether the country that is the claimed source of the meat has inspections or not. And if not, WADA maybe will start advising athletes not to eat meat in these countries.