Back in my pest control days, we used to dust the attics just like that. In order homes, where the ceiling and/or duct work wasn’t sealed, the dust would get into the home.
We used Ficam dust, if my memory is correct, then switched to boric dust later, as it was deemed “safer”.
I have been in many attics with boric dust and immediately show symptoms, sore throat, itchy skin, coughing, etc…by the time I leave the home.
I make everyone aware at the home and in my report as some do have sensitivity to it.
The duct work could be supported, but a lot of the older duct work was just laid on the attic floor.
And the vent fans should terminate to the exterior, preferably through the roof.