As it turned out, airflow isn't the problem; the motor just wants too much juice...
Here's the whistle with its factory wiring harness. Works fine on 12 volts or so or higher.
I broke out an old whistle relay and did a test fitting. Doable, but not with those nuts and bolts holding the trucks in place.
So, I took a page from Lionel's book, and used some eyelets from my sewing kit to rivet the trucks back in place.
You say you don't have a sewing kit? Shame on you! 
Incidentally, these eyelets from Hobby Lobby are a close match to the original Scout rivets.
Wired some leads to the whistle motor bridge rectifier...
Preliminary test: IT'S ALIVE!!!
Repurposed an Erector bracket for mounting purposes.
The blued screw (slightly offset) blends in with the finish.
Test with the relay mounted!
Now to assemble and test...
Long story short: The whistle works fine...as long as there's no engine involved. The motor only works when the locomotive isn't pulling much juice. For example, a bump and go trolley works OK...
...but that's a tad silly, even for me.
Oh, well. The concept works and the air flow seems to be fine with the coffin tender shell on, so it's just a matter of finding a whistle that'll work with the rig.
Mitch
