Would foam do the deed...?

We heat geothermally, but apparently are losing lots of energy because we have lots of poorly insulated ducts running through unconditioned attic space. The ducts run along the floor.
Also, it appears that the ducts are poorly sealed in those runs.
It has been suggested by one insulation contractor that we have them "seal the ducts" and then "blow cellulose" into the area to insulate them.
One hitch is that we need to walk between the ducts to access a small mechanical room that houses an air handler.
It would seem that in order to allow that access, we would have to "box in" the ducts so that the cellulose would be contained.
And so this question:
Could foam be used to both insulate, and seal the ducts?
Are there reasons that foam might not be the best way to go?
Are there other options I should consider?
Sincere thanks for any thoughts on this,

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