What is the point in "burping" a keg right before force carbonating it? When the co2 hits the keg during carbonation it is going to provide a protective blanket to any air above it.
I am confused about this practice and I believe it is redundant. I can't offer evidence because I always burp the keg myself. :-)
If you want to serve cask ale at home with a beer engine and not have oxygen spoil your beer there is a device which can be connected to your co2 tank which will shoot a burst of co2 into your keg each time the beer engine is pumped and oxygen is shot into your keg. This co2 will protect your beer from the oxygen and this way you can have cask ale at home without having to finish it all in one go. The fact that this works should be reason enough that burping is unnecessary.
Here are a couple of videos showing this device:
And here is the device on Micro Matic: Cask Breather