1/4" ply will bend more easily if you orient the grain across
the bend instead of along it. The grain should be parallel to the
spars.
If it's 1/4" birch ply, that would seem a lot of overkill
for a wingwalk. If it's 1/4" pine or fir, it should bend without any
problem. My son and I finished a small boat last year, using 1/4"
Chilean Pine plywood, and we put some impressive curves in it. See this
picture of an identical boat:

http://www.glen-l.com/designs/outboard/dsn-sqtd.html

Dan