I finally found the answer, on my nth read through of the documentation. The reason why my queries weren't working was actually very simple - and also completely illogical.
Instead of using conventional lat/long pairs for the bounding box, Flickr is using long/lat. I have no idea why someone made that illogical decision - it's something that's very easy to miss, as we're conditioned to think in lat/long, so I just misparsed the documentation that Flickr provides every time I read it.
Still, my code's working now (I'll be putting my location/weather/interestingness mashup online soon at my new development web site - www.sbisson.com). I just need to make a few final refinements to the search loop, and then write the tutorial article that I've been developing the code for...
Lesson re-learnt: be more careful when reading the documentation.
And if I ever meet the person at Flickr who made the decision to have an API that reversed common conventions, I'm going to have a conversation about how design by contract needs to respect conventional data formats.