Chuck Westfall (Canon USA) has provided information about the weather sealing found in some pre-officially-sealed lenses.
According to Mr Westfall, "Pre-1999 white EF lenses such as the 200/1.8L, 300/2.8L, 300/4L, 300/4L IS, 400/2.8L, 400/5.6L, 500/4.5L, 600/4L, 35-350/3.5-5.6L, 70-200/2.8L, and 100-400/4.5-5.6L all lack mount gaskets but have moderate dust and moisture resistance for their switches, focusing rings and zoom rings where applicable."
For example, "The EF400mm f/5.6L USM lens was designed before Canon started offering "dust and drip resistant" lenses, so it does not have a mount gasket or a built-in protective filter. However, it does have seals under the switches and the focusing ring, so it is weather-resistant to a point. For best results in wet conditions, I suggest using a clear filter on the front of the lens and covering the lens barrel with a plastic wrap."
"Canon will not retrofit mount gaskets to EF lenses that were not introduced with that feature." I of course had to ask that question.