Your particular use case is far enough from the mainstream that it is IMHO up to you to set the necessary flags when installing such modules to meet your specific requirements.

Cool ... I can live with that ... and thanks for taking the time to express that view.
However, although my use case is not mainstream, I'll point out that it's still (AFAICS) an officially endorsed use case.
And that muddies the waters a little, IMO ... but probably not enough to make much difference.

I assume that there's no objection to my enforcing the installation of my Module::A (Math::FakeDD) into $Config{installsitearch} for all who attempt to install the module.
Someone should let me know if that's a bad assumption.
(There's no guarantee that anyone other than me would want to install that module, anyway.)

Cheers,
Rob

Update: Math-FakeDD-0.05 also now available at cpan.