No, there is no such option. As far as I can see, escaping of slashes is optional. The only thing I can find are suggestions that some real-world implementations require it. All I can suggest is building in the search and replace, like this:
use framework "Foundation"
on convertASToJSON:someASThing saveTo:posixPath
--convert to JSON data
set {theData, theError} to current application's NSJSONSerialization's dataWithJSONObject:someASThing options:(current application's NSJSONWritingPrettyPrinted) |error|:(reference)
if theData is missing value then error (theError's localizedDescription() as text) number -10000
-- convert data to a UTF8 string
set someString to current application's NSString's alloc()'s initWithData:theData encoding:(current application's NSUTF8StringEncoding)
-- remove escaping from slashes
set someString to someString's stringByReplacingOccurrencesOfString
:"\\/" withString
:"/" if posixPath is missing value then -- return string
return someString as text
else
-- write to file
someString's writeToFile:posixPath atomically:true encoding:(current application's NSUTF8StringEncoding) |error|:(missing value)
return result as boolean -- returns false if save failed
end if
end convertASToJSON:saveTo: