> Proposal: an option to convert such URLs into
> "indexname.php&parameter=XYZ.html" instead - this
> would make mirrored URLs corresponding 1:1 with the
> original in a clear and predictable style.
Can't do that because it isn't a valid filename on some systems. 
You can generate things like parameter_xyz.html via User-defined structure -
<http://httrack.kauler.com/help/User-defined_structure>