Don't know if you can land overweight on a Boeing like you can on an airbus, procedures permitting.
Not sure about the -900 but on the -3/4/500, you can if necessary. However, if there is no need to (for example, if there is an engine rundown with no fire), you should burn off the fuel to get the weight down below MLW. If that takes an hour, you can perhaps consider other options as your nearest suitable airport. For non-ETOPS aircraft, this can be anywhere within an hour's single-engine flying time.