Can I use a full, non-graduated ND filter positioned halfway (or another appropriate distance) over the lens to approximate a hard-edge graduated ND filter, or will the transition be too sudden?
The edge will blur out pretty well -- the first Cokin grad ND filter had a hard transition, and I found it worked pretty well. The filter is so much closer to the lens than the subject (unless you're trying this with extreme macro work) that the edge of the filter will be nowhere near in focus.
The problem I'd be more concerned with is the potential for focus shift in the covered versus uncovered area. Using the old Kodak Wratten gels (paper thin, and at one time actual gelatin -- I don't know if they still make them or if they're still gelatin) there wasn't much to worry about, but the thicker plastic filters have a much longer optical path. Even clear glass often shows a change in focus or a difference in image size between the "through" and the "around" side of the picture.
Yes you could do that, but it wouldn't really be a "graduated" filter, it would just be a partial frame ND filter.
Would the transition be too sudden? Probably, but that is entirely dependent on the look you wish to achieve.
Keep in mind though, that this is not going to work well with round, screw in filters. Also, even a "Hard-edge" graduated filter has a gradient, and will be a much softer edge than you will get from a full ND.