LOESS doesn’t have an explicit concept for standard error. It just doesn’t mean anything in this context. Since that’s out, your stuck with the brute-force approach.
Bootstrap your data. Your going to fit a LOESS curve to the bootstrapped data. See the middle of this page to find a pretty picture of what your doing. http://statweb.stanford.edu/~susan/courses/s208/node20.html
Once you have your large number of different LOESS curves, you can find the top and bottom Xth percentile.
