The next question seems to be whether that "sweet substance that the tree shews lap up while sitting astride the pitchers" contains an ingredient that induces immediate activity at the other end of the tree shrew. This might be useful to the tree shrews, too. Any waste materials that are sequestered in the pitcher plant are, presumably, not available to predators as information that there is a tasty tree shrew in the neighborhood. There could be more than one element of symbiosis driving this.
Very good question. Your suggestion does provide a possible avenue for a benefit to the tree shrews, which otherwise is tricky. If the pitcher plant did something to mask the smell, that would enhance that hypothesis.