Well carpenter, I don't know if it could be an urban myth as most septic tanks are in the coutryside, so it'd have to be a rural myth I suppose, but like oops, I've heard a lot of people talk about this.
If you think about it, all the waste going into the tank is from inside the house, and any bacteria in the surrounding soil are going to take a while to migrate into a pretty well sealed up concrete tank. The full decomposition of the material may start earlier because of the introduction and lead to a better performance for the ST.
Perhaps the idea of a badger isn't a good one due to the bones etc., but it's definitely going to be roadkill, but a bird might be the better option alright. Although, having seen up close the pumps that they use, I doubt very much if some 1-2 year old fully immersed in a septic tank badger bones would cause any harm to them, think the contractor sucked up a couple of bricks from my Father in laws tank when he got it emptied last year (for the first time in 30 years!!) Anyway I would feel bad about throwing a good chicken into the tank without it going through me first!!
As for the recipe, I think that could have been Hugh Fernley-Whittingstall, who did a series on how to live on whats around you. Saw a show where some ultra vegan new agers had a roadkill banquet to celebrate one of their numbers 5th anniversary in his treehouse. In another one, he was going to cook some of the pidgeons (or wild rock doves to give them their proper name) from Trafalgar square, but after he realised that they're mingin' little buggers, he decided to go with some snails which he purged with some nice iceberg lettuce before eating, personnally I think rocket would have been the better choice.