OK, I’m a hard head.
FEMA is not funded, i.e., money not appropriated, to provide border security or assistance to CBP. The very moment that FEMA steps in and assists CBP in something that is not part of FEMA’s mission, Democrats and more than a few Republicans will scream misappropriation, someone will go to court, a federal judge will enjoin FEMA from assisting CBP and the whole plan will come to a halt. We know that this administration will not defy the orders of a federal judge, even when that judge(s) clearly exceeds his authority and jurisdiction under the Constitution.
You don’t know that. If DOD buy the equipment from FEMA that could work. They are thinking out of the box which is something hard heads are not known for.