Like LuaHookTranslateName but executed at the map-to-storage phase of a request. Modules like mod_cache run at this phase, which makes for an interesting example on what to do here:

LuaHookMapToStorage "/path/to/lua/script.lua" check_cache

require"apache2"
cached_files = {}
function read_file(filename) 
 local input = io.open(filename, "r")
 if input then
 local data = input:read("*a")
 cached_files[filename] = data
 file = cached_files[filename]
 input:close()
 end
 return cached_files[filename]
end
function check_cache(r)
 if r.filename:match("%.png$") then -- Only match PNG files
 local file = cached_files[r.filename] -- Check cache entries
 if not file then
 file = read_file(r.filename) -- Read file into cache
 end
 if file then -- If file exists, write it out
 r.status = 200
 r:write(file)
 r:info(("Sent %s to client from cache"):format(r.filename))
 return apache2.DONE -- skip default handler for PNG files
 end
 end
 return apache2.DECLINED -- If we had nothing to do, let others serve this.
end