Calling the following function before any tests are run appears to prevent WSGIWarnings being issued by WebTest. Now at least anyone running my tests will not need to manually edit WebTest ...
def addSEARCHToWebTestValidMethods():
new_valid_methods = list(webtest.lint.valid_methods)
new_valid_methods.append('SEARCH')
new_valid_methods = tuple(new_valid_methods)
webtest.lint.valid_methods = new_valid_methods