A breve over an "s" is indeed unusual. I am assuming that's what you mean when you write "a half-circle" above it. Look carefully to see if it is a caron or circumflex.
If it is an breve, then you can use the UEB modifier for breve (dots 4, 346) ahead of the "s". If you want it to be a single-cell character, then you could chose the UEB "the" symbol perhaps, and mark it on your SS page that it represents an s with a breve over it.