On a stretch of eave where there is no flashing at all yet, the roof deck is higher than, and back from, the fascia. If gutter apron rises straight up from fascia front then back (on an angle) to roof deck, it would have to hold up shingles for a space. The gutter aprons I've seen offered on the internet are not particularly heavy gauge. Could they support shingles for an inch and 1/2 or so?
A local animal control company could install a very heavy duty aluminum flashing that is made especially for them--the same angle as regular gutter apron but with no kick-out at the bottom, and no flexibility. It would certainly hold up shingles and keep out squirrels as well, but I've read that the lack of a kick out would pose a problem of capillary action for water. Also, with no flexibility, I'm thinking it might not be good if the angle of the roof is different from the angle of the flashing.
The only other local source of gutter apron that I know of is Home Depot, and I think theirs is both relatively light gauge and lacking a kick out.