How can I override the default values for a parameter in a report?
It appears that SQL Reports actually does what I want automatically - but only if the parameter with the dependent query immediately follows the first query.

We were modifying the report for other reasons and moved the offending drop down higher up the parameter list so it was second (after the query that filled in the location drop down). Having done that, the drop down was populated automatically.

The blog post I linked to in the question mentions that parameter order is important and there's a separate post about that by the same author.