All X and (no) Y

Just stumbled across this entry on Language Log, pondering the Economist's barbaric use of 'All mouth and no trousers" in a recent headline (as noted below):

This is apparently a UK expression that I've somehow managed to miss. The deck ("Are foreign firms as keen on Asia as they claim to be?") and the rest of the story make it clear that the meaning is same as "all hat and no cattle", "all sizzle and no steak", "all bark and no bite", etc.
But the mouth v. trousers variant is puzzling. The X in "all X and no Y" (hat, sizzle, bark) represents pretense, and "mouth" is fine for that. But the Y (cattle, steak, bite) stands for the corresponding reality -- and what's so real about trousers, especially in correspondence with mouth? None of the obvious answers belongs in an Economist headline.
A bit of web search demonstrates that the expression is a fairly common one, with variants like "all talk and no trousers", "all hype and no trousers", "all puff and no pants", etc., but doesn't explain where it comes from.
There's some evidence on the web that others are puzzled about this as well -- one reviewer on amazon.com says that the (rock band) Crimea is "posturing, all trousers and no action". So apparently there's a stage of substance even beyond stereotypically male garments.

A correspondent then points towards this humble blog, and the unfortunate confusion surrounding 'trousers'/'no trousers', prompting this comment:
Aha! Wow! etc. We should return the favor and re-metonymize some of the "all X and no Y" expressions to remove the negative: "He's all hat and cattle. She's all bark and bite." It works even better, I think.
Which seems a bit silly, really.