I agree, this is probably a right-handed persons writing with their left hand, who had some formal technical drafting training. Remember that Atta guy and his terrorist cell in Hamberg, Germany studied urban planning. Urban planners have to go through architectural drafting classes to develop acceptable lettering. Also urban planners usually use thin felt markers or technical ink pens for their drawings. A technical ink pen would be too destinctive of a clue back to an architecture or urban planner, so they stuck with the thin felt markers, available everywhere.
I would say look for a Jr. College in the Trenton, NJ area that has a drafting department. Princton U. just down the road has an extensive engineering department.
The thing I really don't understand is why they didn't use a printer (especially an ink-jet, which naturally blurs enough to obscure any clues that could identify the individual printhead).
I think that pretty much rules out anybody who expected to still be alive when the letters were delivered (yes, they were mailed after 9/11, but arranging a dupe to drop the letter into the mail on a prearranged date would be trivial).